The problem that remains in Mountain Lion is that you can only use 1 display at a time when in full-screen mode.
It's better than Lion in that you will be able to choose which of your displays is used to go into full-screen mode, rather than always just using the 'main' display.
Disclaimer: I haven't actually tested in Mountain Lion, but this is the only consistent explanation I've been able to gather from careful reading of people's complaints and Apple's description of the feature change.
http://news.ycombinator.com/item?id=4235432