I recently withdrew part of my holdings from Poloniex, they are very legit in my opinion and may be the best all around cryptocurrency exchange but it would really suck if something like Bitfinex, Cryptsy or Mt Gox happen to Polo. No one is really safe.
For traders and investors is difficult to not have all their firepower on a exchange, I have more than 5 currencies at the moment, some don't even have working wallets. So it's not that people are just lazy or stupid.
We will not still dont know what trading site are will stay alive for a long time.. even its a poloniex we dont know what will happen with poloniex..
It can also be down in the future if that site is already been monitored by hackers..
Hackers has high technology tools for hacking where all data base will be recorded as daya cookies that they can convert them into phrases.
So if you stay huge amount in trading site better to get those and just deposit a few bitcoin depends if how much altcoin you are holding there..