I do not believe how people are taking short positions. Almost all the Moneros from lending markets are now employed.... 
I have some out. 0.7%+ per day/21% a month/255.5% a year. With these rates traders won't be short for long. Sideways for a few days or a minor increase in price will cause most of these short positions to be closed.
I've started to do the same... it's too attractive not to get a piece of the action
This trend will make Polo increasingly juicy for a hack / inside heist. Admittedly and irresponsibly I have not looked deeply into their security or insurance situation.
Only what you're comfortable losing on exchanges...
The way I see it it is generally Bitcoin being stolen so the Bitcoin I keep at exchanges I buy other coins with and if not I withdraw them no exceptions to that rule. Stealing all Monero from Poloniex when the only liquid market is Poloniex is a frivolous pursuit and if you find a hole in their security and use that opportunity to steal Monero before Bitcoin you are too retarded to find that hole in the first place.