There's only so much to that productivity argument. I suspect the time it takes to make big changes to the Rails stack for performance sake far exceeds the relatively small savings in initial development time. According to Compete, Twitter's not that big [0].
Yeah. It's a give and take to be sure. I'd take Twitter's current position any day. But you're right they're still in a position to stay ahead of competition and net productivity of development and scaling has to be considered.
Worth noting Twitter's audience is using it a lot differently than Techcrunch's is, so relative audience size is not totally meaningful.
Most of the people I know don't use their web interface to post. They either use AOL IM or most likely SMS through heir cell. That's why twitter.com's number are probably way off.
[0]http://snapshot.compete.com/twitter.com+techcrunch.com?metric=uv