I did a steady, long climb (well for around here anyway) last night. I paid special attention to my pedal stroke and what I was feeling just for this thread.

What I noticed was not a steady push-pull. The push was still 95% of my effort, but every minute or so, I could feel the pull get stronger for just 3-4 pedal strokes, but enough to give my quads a tiny reprieve.

To illustrate the importance of that brief rest and the good it does, take a heavy book and put it in the palm of your hand. Stretch out your arm straight to the side and hold the book up as long as you can. When you can no longer hold the book up, rest you arm for 4 seconds, then put the book back out. If you were at a "10" when you had to put the book down, you may be at a "7" when you put the book back out, after just a few seconds reprieve.

Now repeat that several times up a climb and you can see how clipless lets you maintain the same power for longer.