i get that some people want to keep money on a exchange in maybe USD OR EURO so they can buy whatever coin they want fast [...]
There is no need to use an exchange. Many of them have ridiculously high fees (mostly withdrawals) and force their users to pass through KYC verification. I would rather use Changelly or some other third party service which offers quick exchange between different cryptocurrencies. Even TREZOR started supporting these kind of services natively in their web wallet interface.
What if you want to buy BTC with fiat money? What if you need to sell for fiat to pay bills or rent, or just need to hedge against a bear market by holding cash?
Changelly and other altcoin exchanges won't help you there. Holding USDT or other "stablecoins" exposes you to 3rd party risk like any exchange does.
The P2P market where I live is not very liquid, and KYC is getting more prevalent there as well -- at least with LocalBitcoins traders. Exchanges are a necessary evil, for now at least.