In practice I wouldn't say that's ether's main use. If you add up the total gas fees so far, it'll probably amount to less than the $150 million in ether raised for TheDAO, the $5.5 million raised for Digix, etc. And if you look at the code samples on ethereum.org, one is for a crowdfunding app that raises funds in ether.

And of course, if you're paying miners with ether, then ether better be a functioning currency or it's a worthless payment.

You're certainly right that Ethereum has lots of uses beyond simple value transfer, but if you want to transfer USD instead of ether, you need to trust someone to back tokens with USD, and trust governments not to shut them down over KYC issues.