In a nutshell, it all comes down to being unable to back out if something goes wrong. For example, you buy a few bitcoins at a December high and expect the price to continue rising, which is kind of obvious. Instead, the price starts crashing down and you find yourself in a situation that you didn't envisage or consider beforehand. So your best option would be to bring things back where they were as fast as possible even if it means some loss.
It depends entirely on how much you have staked in your trade. If you have gone all-in, then yes, you need to sell and take some losses in order to re-enter a trade at a different price point.
But if you have staked just 1/10th of your stash, you can afford to wait it out. Lend out those coins on Poloniex while you wait for the price to recover, and use the rest of your stash to continue trading.