You won't find very many people that have been doing this for very long that think keeping more than necessary on an exchange is a good idea.
If you think it is, you just haven't been doing this long enough. Learn to manage your own security and wallets.
Yeah, unless you have your coins in your own personal wallet then you do not really 'own' your coins, the exchanges do. Anything could happen to them, which is why you should always keep them in cold storage.