That's true this is one of the reason why I seldom use my account,because they don't have all the coins I'm trading from other exchanges but I believe hey are top exchange I read a lot of positive things about this exchange and every coin's goal is to get here.
A considered top exchange that already established a good name in the world of cryptocurrency will never list all altcoins in an instant. Poloniex have standard that coins must have in order to be listed in their exchange.
That's why as you can see, once the coins was listed in Poloniex, it will turned that coin into a stable one that worth to trade. While having such features like that, they are also delisting coins that doesn't established a decent volume in a certain period of time which is good for altcoin traders to stay away from those coins that will turned to a shit one.
thats a good points to suggest since shitcoins can turn bad to his exchange and instead of people would love it they will spread some fuds due to rage since they doesn't want the scam listed on exchange,
He should do some deep analytical background checking so that he would list the good coins, a voting system like c-cex could help in his wants.