Leg extensions, like any other isolation exercise, can be a useful
tool. The are good for re-hab, training around an injury, bringing
up a lagging part, and balancing out a routine. They are no subsitute
for big compund exercises. For a beginner and intermediate lifter
with no injuries, use of isolation exercises should be limited. As
you become more advanced they become more useful to maintain a body
part while focusing on a lift that may not provide sufficient
stimulus to a given body part. As an example, you wish to spend the
next several weeks focusing on your deadlift. Adding the leg
extension to your routine will work your quads more than they will get
with just the DL, but not cut into your recovery to any great extent.
Some find this use of leg extensions useful. YMMV