Now this part I agree with. With all these hacks going on and risks of 51% attacks is it worth holding coins on exchanges because of the high risks? I would say do not take risks.
As long as it takes syncing blockchains and as much space as it occupies on your hard drive it might be better to keep your most valuable coins away from exchanges, for now at least.
The only reason to have funds on an exchange is to trade. If you finish trading - remove the funds.
Also check the withdrawal fees before depositing. Some exchanges have horrendous withdrawal fees.
Sometimes it is even worthwhile to exchange your BTC or coins to something like Doge to avoid paying excessive withdrawal fees. I often used Doge for sending between exchanges. The exchange rate is reasonably stable and the withdrawal fee usually quite low. Most exchanges have a Doge market.