Because eventually the altcoin is to be exchanged with bitcoin, which will be done at polo so it makes sense to receive the alts directly there.
Same goes for buying altcoin too right? I can buy alt with fiat from someone and give them polo address to receive. Has anyone done this?
You should seriously revise your response :
https://www.reddit.com/r/BitcoinBeginners/comments/4abehg/is_it_ok_to_keep_coins_and_other_cryptocurrencies/
https://www.reddit.com/r/ethtrader/comments/44zdzm/do_people_store_ether_on_poloniex_or_transfer_to/
https://www.cryptocoinsnews.com/cryptocurrency-exchange-poloniex-insecure-security-review-claims/
https://www.cryptocompare.com/exchanges/poloniex/reviews
https://forum.z.cash/t/sending-payments-to-poloniex-t-address/5899
I don't think that you need more then this.