It's not about doing stuff in platform A that can't be done in platform B. It's about delivering things faster than your competition, something that can't be done if you use the very same tools they use. You can, of course, be wrong and go with the wrong tool, but remember - it's a bet on a technology differential.

The Twitter problems stem not from the web framework they used but in flaws in the design of the data structures under it. When doing something like Twitter, one should never even consider using relational databases for more than a prototype.