People are right that you require a good pedaling technique so you don't lose energy bobbing up and down. But then again, that pedaling technique is basically limiting the amount of force that you do to pedal. During my commute to school, I find that whenever I try to sprint from a red light to keep up with traffic, I bob up and down and end up deciding that I need to accelerate slower.

But when you have a lockout, you can actually sprint standing up without too much of a problem as I have noticed with stiff bikes.

Just think of it this way, advantage of suspension is comfort and traction (as the wheel stays on the ground as much as it can but not much of a difference) while it's disadvantage is losing energy due to giving a lot of force pedaling while trying to get the aid of gravity (thus bobbing up and down laterally). Having the fork stiff using a lockout will end up the converse of that.