I would never ask a for-profit company to open source their core software if it doesn't meet their business model (sell support, etc.). Here's the thing though: I don't care one bit about how they process tweets internally.

However, the "bit of code on the periphery" that Twitter has released is far more interesting to me. Bootstrap lets me prototype web UIs much faster, for example, and who knows? Maybe some day I'll end up using their contributions to MySQL, Cassandra or Mahout.

So, if only for purely selfish reasons, I say kudos to them.