Withdrawing is not necessary for trading.
But if you have a substantial amount you want to be free to withdraw whatever you want .
And now Poloniex have substantial issues.

http://www.newsbtc.com/2017/06/24/users-cant-withdraw-funds-poloniex/

Also support is non existent.
Is working at full capacity.
Something will happen there first or after.

This is my opinion and you are free to use Poloniex if you are happy with it.

Regards