Well, I tend to take a pragmatic approach about this. The question is, how much you want to deviate from convention of underlying framework when developing? 90% of Web applications out there, can reliably stick to rails conventions and they will be well served by it. But from my personal experience - handling file uploads, processing them is often very different from app to app. What we need is composaple modules not a single monolithic library that does everything and there lies my gripe.
