This would be a huge thing in places where there isn't a good free bank account-to-bank account money transfer app. which I assume is why facebook is also eyeing such things.
Only if they have easy access to Ethereum tokens, which is doubtful. Bitcoin is a lot easier to obtain than Ethereum is, usually people have to buy Bitcoin to buy Ethereum, that's a lot of effort just to use an app.
No you don't 'have' to buy bitcoin to buy eth. Coinbase, which is known as the largest fiat-> btc/eth platform allows fiat -> eth directly. But yes, I agree that easy access to eth or btc is not available at the moment for those outside the US or outside the neckbeard community
It's just as much about the liquidity/depth of the ETHUSD market, which is abysmal compared to BTCUSD. Crypto currencies are a great idea, but without liquidity it's fairly useless as a value transfer system, as you'll lose significant sums to slippage when exchanging.