I am, too, disappointed with the plastic mount. While I know its probably good enough, given the metal mount on the actual body I don't see how it would hold up over time if you change lenses frequently.
I'm not troubled by plastic mounts for the smaller, lighter lenses. While I don't use them as often as lenses with metal mounts these days I've never had any problems.
If nothing else, they reduce the wear and tear on your camera body. You are unlikely to damage the metal mount on the camera with a plastic mount on the lens. I'd rather need to repair/replace my 55-250 than my 7D (although I don't think I've ever used it on that body).
Personally, I feel very nervous about plastic mounts and for that reason I've never purchased the 55-250 even though it was better value for money than the 70-300 I bought and sold later. I often hold the setup at the base of the lens with only a part of my palm supporting the body and I'd never be comfortable doing that with a plastic mount. And metal mounts don't significantly wear the body. I've seen many old bodies (including my own EOS 650) with many lens changes and the wear is miniscule.
By the way, what surprises me is that Canon still makes and sells the old, crappy 75-300 and even more surprising, even people using Rebels buy them!
EOS 5DIII, EOS 6D | Rokinon 14mm f/2.8, TS-E 17mm f/4L, EF 24-70mm f/2.8L II USM, EF 35mm f/1.4L USM, EF 100mm f/2.8 Macro USM, EF 135mm f/2L USM, EF 70-200 f/2.8L IS II USM, 1.4x III, 2x III | 600-EX-RT x3 | EOS M + EF-M 22mm f/2