A lot of companies, including Philips lighting and BLV Licht, offers dedicated electronic ballasts for their horticulture HPS lamps, despite they can operate without problem on a regular HPS gear of similar wattage. This despite 100hz flickering usually isn't a problem for plants and that electronic HID ballasts above 250W, have higher losses than magnetic ballasts.
Why electronic ballasts are so popular in professional horticulture lighting?

If I'm being cynical, which I am, I'd say it's because their customers don't know any different and they can charge more 

Many grow lights are mail ordered, and electronic ballasts cost less to post than magnetic because of weight (and size, too). It might be cheaper to make a dimming electronic ballast as well, but I'm not sure on this one.

The profit hypothesis probably holds some truth.

They also come in nice packages so they are simple plug and play, they have built in fan cooling, which while that seems like another thing to fail when you have thirty or forty of them in a single room it can get pretty warm. For the most part hydroponic operations like to remote mount them away from the water/moisture. They also offer ballasts which you can switch from HPS to MH so depending on the stage of the plant so it's a simple re-lamping and flipping a switch vs re-lamping and re-ballasting so it's time saving and just about anyone can screw in a new bulb but most people wouldn't know how to rewire the ballast. Some ballasts can also overdrive the lamp so it outputs more light, and when you don't care about the lamp life and your goal is maximum light these are about the only option, plus they can survive larger voltage swings before the lamp drops out so when items like fans kick in or A/C compressors you don't lose lighting.

Don't kid yourself Hydroponics users are not typical users they do look for different things then most users here would look for and solid state producers are filling that need, and for the most part they know what they are growing so money is not the issue if the ballasts don't last or the bulbs don't last they know they will just buy more..

A trip to a hydroponics's store is interesting, you will be hard pressed to find a core/coil ballast, the one I stopped at only had three magnetic, 1000W MH / 600W HPS /1000W HPS but they had over fifty solid state ballast and they have neat lights like green T5 HO 2/3/4 foot florescent tubes for night lighting (by sunblaster, they have red and blue as well.) which is what I was after but the green ones were sold out, so I left empty handed.. 

My curiosity was piqued, so I had a look at a couple on eBay, they look nice and some are switchable between different wattages, say 250/400/600W, which would be very useful if you have several wattages of lamps you want to fire up occasionally. They do seem to be a bit on the expensive side, although I suppose in comparison to separate 250, 400 & 600W sets of gear they probably work out cheaper.

Out of my price range right now, though 

One reason was mentioned: The ability to change wattages in service.
Other reason is related to their mass: Because they are lighter, they may be installed in the fixture body (without overloading the support structure, as there use to be huge amount of them installed) and so allow way simpler wiring (no need separate cable from the control cabinet to each individual lamp).
Mainly the savings related to the second may be easily greater than the extra the difference ballasts use to cost.
Another plus could be way more advanced EOL protection, so less likely for the lamps to rupture and so contaminate the products underneath. Again could be translated into money savings, mainly in professional industrial facility...

One reason was mentioned: The ability to change wattages in service.

I'm curious now, what would happen if you switched wattages on the ballast while a lamp was running..? 

I'm curious now, what would happen if you switched wattages on the ballast while a lamp was running..? 

If you turn them down, the light dims, if you turn them up if gets brighter and if you turn them way up the EOL protection circuit kicks in and the lamp turns off, I've seem some with a rotary dimmer switch on them and you can use it like a household dimmer on a regular light, now I don't know if this will affect the light life and color quality but they are fun to play with in the store and it's not my light so I'm not worried about it only lasting a few hours if it's over driven..

Thanks for the info, we may invest in one should we get any higher wattage lamps, we only have the smaller wattages at the moment, still building up the collection 

I'd visit a hydroponics store if you have around you and get them to show you what they can do, most of the people working there have them setup and are more then able to show you how they work and get he model number and look on a website like Aliexpress for the most part they are about half the price or better then getting them locally (that might not be the cheapest or best it was from a quick search but not bad for 76 quid brand new with free shipping for a 400 watt HPS/MH dimming ballast)

I'd visit a hydroponics store if you have around you and get them to show you what they can do, most of the people working there have them setup and are more then able to show you how they work and get he model number and look on a website like Aliexpress for the most part they are about half the price or better then getting them locally (that might not be the cheapest or best it was from a quick search but not bad for 76 quid brand new with free shipping for a 400 watt HPS/MH dimming ballast)

I've had a look on Google maps and there is one not too far away, so we'll go and take a look. Looking on their website, the ballasts are a bit expensive there, but I did see one on eBay (from a seller in Scotland, no less  ) that looks reasonable for the price. We're a bit short of funds right now, but maybe later in the year.

A trip to a hydroponics's store is interesting, you will be hard pressed to find a core/coil ballast, the one I stopped at only had three magnetic, 1000W MH / 600W HPS /1000W HPS but they had over fifty solid state ballast and they have neat lights like green T5 HO 2/3/4 foot florescent tubes for night lighting (by sunblaster, they have red and blue as well.) which is what I was after but the green ones were sold out, so I left empty handed.. 

I used to see the Red/Green/Blue F54T5 HO's on eBay. I got a few of them just to check 'em out .. even have a red & blue in use (along with 2 standard F54's on my houseplants).
They don't seem that easy to find anymore.

I've thought that it would be interesting to go in one of those 'grow' stores just to check out the lights they sell.
But always figured they'd be way overpriced.
Plus pretty much anyone (here) going to them is probably growing a certain type of plant that I don't have any of.

I used to see the Red/Green/Blue F54T5 HO's on eBay. I got a few of them just to check 'em out .. even have a red & blue in use (along with 2 standard F54's on my houseplants).
They don't seem that easy to find anymore.

I've thought that it would be interesting to go in one of those 'grow' stores just to check out the lights they sell.
But always figured they'd be way overpriced.
Plus pretty much anyone (here) going to them is probably growing a certain type of plant that I don't have any of.

Actually I was in one last night, some of the stuff is on the high side, but they have everything out in the open so you can check it all out, but some of the odd stuff like the green 54 watt HO's you can't find else where and getting them shipped is a pain so if they are a few bucks more I don't mind and they don't arrive busted, and some of the places even sell things like used ballasts and they can be a good bit cheaper, and not every one grows that certain plant I know a lady who grows just about everything but that plant, she even has a pineapple and coffee growing under lights in the house, but yes there are people that do as well that visit those stores, and people like me who can kill spider plants...