A classic use case is engaging in commerce (whether it's with ETH, DAI, USDC, etc), without needing to trust a middleman like PayPal to not freeze or steal your funds. For this one needs ETH to pay gas.
Common: The middleman is still there in the most common case since most wallets are custodial. There are a ton of recent examples of exchanges freezing accounts, including the very article that we're commenting under, so I don't really get what you're talking about.
Good: It doesn't make sense to use a wildly volatile deflationary token as currency. Deflation discourages spending which is why there is no real currency that restricts the supply of money to a fixed amount, because it encourages hoarding.
I wouldn't argue that everyone should self-custody. If many users have access to convenient custodians who they can trust not to steal their funds, that's great; it's just that not everyone does.
I don't really have a stance on Eth's monetary policy, I was just trying to address your point about a use case.