To quote the Big Lebowski, "That's like, your opinion".

Python 3.3 is a better Python.

And what "great cost" does 3 burdens us with anymore?

Since most of the lib porting has been done (it's been 5 years out already), there's not much cost to be beared, if any: you just get to use it.

Many useful libs still aren't ported and some likely never will be (like Twisted). But more importantly, there is a vast amount of application code out there not ported. If you're working on a py2 project it's even hard to start new projects with py3, since you get no interoperation.

Breaking compatibility should have never been done without being able to run both py2 and py3 code in the same process.