Not only does it use some of your energy input to go up and down instead of forward, it is kind of annoying when you stand up to pound up some nasty little stretch of hill and your forks are bobbing up and down, especially if you are on sealed surface at the time.

That being said, I rarely, if ever, lock out my fork on either bike (even my urban commuterised hardtail - you never know when you are going to come across a kerb you need to huck) and on the odd occasion I do have the presence of mind to think to lock it out (it would have to be for a decent climb I knew was coming) I almost always forget to unlock it until I am in the middle of some hairy descent and wonder why it seems so ugly!