Originally Posted by contango

If the suspension is set so it's soft, as the back wheel pushes you forward some of the energy goes into bouncing the fork. I remember the first time I rode my (then) brand new MTB up a hill - I was fat and desperately unfit and really noticed how as I struggled up a hill I could actually see the fork bouncing. The trail wasn't particularly bumpy, but some of the energy that I wanted to be pushing me forward was just bouncing the fork.

A suspension fork with a lockout seems like a handy compromise, but if the closest you're ever going to go off-road is when you cycle on the pavement/sidewalk you don't need it.