Ability to save app data when swapping apps out of your phone.

So I beat all the levels on angry birds a couple weeks ago, which was nice because i have hit a very busy time in my life where i just can't be playing angry birds any more.

I would like the following feature which I am pretty sure does not exist in ios/itunes currently, but correct me if i'm wrong: Why can't I save this app data when my phone backs up (preferably in the app itself)?

with wireless syncing it is easy enough to swap apps in and out, especially while on wifi. In particular some games (though I only own a couple) take up tons of space. Or maybe I just don't want to have some sensitive app's data on my phone for the next few months until i need it again for work or something... So how about let me delete it from my phone knowing that it is sitting on my itunes in the mean time?

Basically I want to delete angry birds and load it up again in the future when my life gets so boring that i want 3 stars on everything.