it is like walking into a shop in US and want to pay with Zimbabwe dollar instead of dollar that everyone else is using.
when it comes to altcoin trading, majority of people are doing it to increase their bitcoin holdings. so they all trade with bitcoin to make profit on that also. this means for any altcoin the BTC market is more popular than anything else.
you still have other options, you can buy most of the big coins with fiat directly. there are multiple exchanges that have the altcoin/USD, altcoin/EUR, ... listed which you can use.
pros? maybe if you want to start without having bitcoin you can skip a step and pay 1 less trading fee. it is not that big, so it is not such a pro!
cons? you will have to go to a specific exchange that has that market, not all of them have it. so your options is limited. you can't buy all the altcoins with everything you want. and as i said these markets are less popular, not all of them but in most cases you may even see less liquidity.
or you can just get usdt anywhere
