I can see that it may appear like that and for small amounts you may be happy to leave on an exchange. But in reality, although you may have had the opportunity to dump at the peak, this still supposes you know it was the peak.
Plus, you only have to look around to see all the issues with polo freezing withdrawals again to see why people (myself included) are much happier keeping my coins under my control. I may have lost the opportunity to dump my new coins at an artificial price, but I know I have the opportunity to dump my BTC at a time of my choosing, not when the exchange decides to allow me to have access to them!
I personally chose to keep my coins myself as well, I had considered sending some to Bittrex or HitBTC, I didn't trust hitBTC and in the end decided not to send them to Bittrex but people who did use Bittrex have ahd control of their coins sooner than the rest of us who control our private keys and faster than myself who chose to keep them on a trezor. Personally had I had the opportunity I probably would have sold mine at $1100 and then bought back a day later at like $400 but I'm not hugely concerned over it and I'm not smart enough to figure out how to get them off my trezor at the moment so for the time being I'll leave them be.
Also, I should add having funds frozen was the reason I ddin't trust hitbtc, when I tried to do a little research on them I kept comming across articles that talked about them freezing peoples Bitcoins.