Yeah, I agree, I've had tons of plastic bobbins with worn edges and I throw them out.
With a lot of the bobbins - metal and plastic - there are several levels of quality you can buy. Guess which level most people stock?
I've never bought the cheap ones and haven't had any trouble with the metal or plastic. Now, that's me as a "dealer" (i.e buying wholesale) and I can see the different levels and choose accordingly. The consumer has a hard time doing the same because they have to buy what's offered. All I can say is looking for the best price will bite you in the rear as far as bobbins.
This comes back to the consumer though. We've demanded a less costly solution and manufacturers have responded with a cheaper product. Manufacturers have told the factories that the lower quality level was acceptable for their client (us) in order to supply product at the price we're willing to pay. The only way to change that is to stop buying the lower quality and demand the higher quality products.