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?
It is wise if you plan to sell right away, I'm doing same with digibyte. I earn trough playing various games and my withdraw address is poloniex, I never sell all, I have some little amount there just in case. Maybe that is not wise but I don't risk too much, that is better then to download wallet just for some coin. I have some other coins that I hold there, nothing much, but I trade so I sell often and buy often.