If you wish to hold some coin then use exchange only for during exchange. Just exchange your coin and transfer on your personal wallet that you have controlled. Specially you can use hardware wallet.
Because many exchange hacked buy hacker. Many people loss their capital by hack.
So please don't hold your coin on exchange for long time.
Holding your coins on exchanger is fine but the only problem is that, if the problem occurs in their own system then your coins might be stolen from you and that is the reason why i prefer placing it into the wallet.
Yeah whenever they can be a scam, with reason their system is hacked, as happened in bitfinex, mt gox and so on, from that we know that keeping all funds in exchange is not the right idea, maybe it's not a problem to hold just to do trade only, but do not keep all your money in exchange dude