We don't do this with other languages so why should we do it with HTML?
It's pretty much been the case with CSS - CSS2.1 should never have been a 'version', and in fact I think it is now retrospectively properly known as CSS2. There were always moving changes in the 2/2.1 spec, CSS3 is just a convention/buzz-word - it just means it's still CSS nothing new to actual error recovery or the basics - but it's now modularised ready for the introduction of new functionality - some parts are implemented/supported some aren't. I think that's probably the intent with HTML5, as someone else said it's too big to inplement all new things in one go, but a bit at a time is good for competition.
If it's any consolation I think, in hindsight, that this way helped the CSS cause, browsers did "compete" to provide support for the more popular/used parts and use cases were tested properly and patched (IOW specs were written/changed live!) instead of theoretically. If certain browsers had not stepped up to bat and tried things, they would never have got done!
A living spec, cuts out the theory (arguments & bickering over who's right too) and gets on with the job quicker, if you like
don't know if HTML (5 or not) needed such a radical proclamation of that same methodology but I think that's it intention.. it should be good news for those that actually want it to start happening anytime soon.