Plastic Reflector is produced for cheapness and economy. I have one street lamp with plastic reflector. In addition, I can say that the production of plastic reflectors is very unsuccessful. Because as you use it, black begins to form and it causes big complaints. Plastic reflector as I pictured it. It came out broken in that corner and I forced it to stick with glue.
Used condition:
https://www.lighting-gallery.net/gallery/displayimage.php?album=search&cat=0&pos=30&pid=185922
I've never seen blackened plastic reflectors, except only one, clearly very inferior way processed (likely the passivation coating was missing at all, so the aluminum reflective layer oxidized) product.
On the other hand "milky"/yellowed plastic lenses are a common sight (not present with just a clear glass in front of optics made all around faceted reflector), the aluminum surface turned "milky white" as well (so instead of redirecting the light where needed, it was just diffusing it), not even speaking about rusted steel reflectors.
Yes, plastic reflectors are rather fragile, they do tend to break if the fixture get some nasty shock (such it breaks the lamp as well, but I understand the broken lamp isn't that big of a problem as the broken reflector is). With quality lanterns/fixtures, the reflector is mounted on some flexible supports (the socket then should be fixed to the reflector, to maintain the alignment), which then should be able to absorb the sharp shocks. Mounting that reflector directly into the metal frame is not that good idea.
So overall fixture construction, when going cheap, the aluminum reflector is a solution. When going high quality, the properly designed, made and installed plastic is the way to go, even when the cost will be somewhat higher.
