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.
The devs are so badass, they constantly mention that they will allow miners to accept any currency for gas fees if it is feasible. Most of us who have some Ether are comfortable with this move too, ironically our hedge for this scenario is to hold some Bitcoin.
Because, Ethereum will prosper not when Ether has a high market cap, only if it can become robust enough to provide a decentralized platform to run code for a variety of apps and services.
True but they're not planning for currency-agnostic fees before they move to proof of stake, which will require ether for staking.
It might not be a done deal, either. A few weeks ago Vlad Zamfir, the lead researcher for PoS, tweeted that he thought agnostic fees were a dumb idea, because they'd be too much trouble for miners.
A lot of contracts are easier to write if you assume a single currency. If you're doing that you have to pick one, and ether seems like the obvious choice.
I mostly agree with your last sentence but a high market cap doesn't reduce Ethereum's robustness either; in fact it makes it more secure.
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.