All my Steem is used to powerup here on Steemit. It effectively locks your money in for a period of time and means that if my account was hacked, they can't get hold of my Steem immediately. I can't sell immediately either but that's allready stopped me selling some at a much lower price than it is now. Then there's the bonus you get from upvoting your own posts, that alone would make me keep my funds here rather than use a 3rd party wallet.