GData Optimistic concurrency

Google Data APIs beat Atompub to the punch and stamped legitimacy on REST. I hope Atompub will end up looking as nearly like GData as possible.

Of note: this is a Hi-Rest API, and now, we can safely say, it's the way the web actually works. Not just the way some very dedicated people aspire for the web to work.

So enough rejoicing. Let's begin analyzing GData to death.

First technical question: optimistic concurrency: What's wrong with plain old HTTP if-unmodified-since/if-match? That's how you guard against concurrent updates in HTTP right? Why invent a new protocol for that?