I don't think it's as usefull as it seems, however. First, you cannot say "this type of units go here, this type go there". So you end up with all your units in a big pile. And usually by the time your units arrive, maybe your objectives have changed and you'll have a hard time tracking who is going where.
Of course, this is because I play mainly huge maps ; on tiny maps, the time needed to travel may not be such an issue. And then, when you have railroaded every tile, it might become more useful. But still.
Then IMHO it's dangerous to send them automatically in enemy's territory, because single units are not as fearsome as a good ol' stack of Death, meaning the enemy will have a great time killing your units one by one as they arrive.
A continental rally point by type of units would be much better ; no need to select a city, no worry about "if I switch my production then my rally point is gone", just say "every tank produced goes here".
Of course, this is really nitpicking. Firaxis didn't have to implement a rally point, and it was nice of them to spend time on it. It's just that it feels unfinished to me.