That's exactly the point. Which version of Gmail are we on right now? 477? Does that feel different from 476? I don't know. I just get performance improvements and new features as they come out.
Mozilla was sick of defining features for a given version and then delaying releases for months because of too-ambitious goals, so they switched to a whatever-is-done-by-the-deadline system. Version numbers aren't particularly meaningful.
I also like build numbers like Microsoft and Apple give, you know if OSX 10.7 GM seed is different from the GA release by looking at the build (e.g. 11A494A)
Personally, I think the time for version numbers should probably be over altogether. At this point I feel it's more productive to just tack a date string onto the thing and be done with it. For example, Firefox 2011-July is way more informative than an actual version number.