I like plastic cameras, especially ones with plastic lenses. Why? Because I like vignetting. I like it a lot. In fact, I like it in most circumstances when I'm not trying to take serious photos.
Cameras like this are for those of us who want to unwind and use something that we don't have to think about using...you just point it and shoot it. There's no focusing, aperture setting, shutter speed, etc...your job is just to compose the photo. It's the same concept as a pinhole, really. The only difference between cameras with plastic lenses and a pinhole camera is the fact that the exposure times are shorter.
I've shown good photos from my Lex35 before. Memorable, usable ones. Truly, you use what you feel like using to make the images you want to make. Nothing wrong with a plastic camera.