There are obvious pros for exchanges like attracting user base that would be otherwise not interested in the outlet. I used myself 2 or 3 exchanges that I would never registered at if not some shitcoins listings. On the other hand this reduces the liquidity of some other pairs coz speculators split up a lot if there are more options to trade. Simple example, 100 mil capital on the exchange, which do you think will be better for algorithmic strats, the one with 20 or the one with 100 coins? Big names will stay away from shitcoins the same way big brokers don't offer questionable assets. There are special brokers for penny stocks and it's always been like that.