catsinspace :
I was of the mind that I wanted to see high frame rates (via FRAPS) so I left the v-sync off. But that was faulty thinking. Most monitors have a 60hz refresh rate, so a game running over 60fps is almost useless and can cause screen tearing. I started using v-sync (capped at 60hz although my monitor is 75hz). All the frame rates below 60hz were still there, but the v-sync starts to cap and match the monitor refresh rate with the game fps after 60fps. It brought my temps down 2-3 degrees C and I'm very pleased. Your eyes/brain can't really notice anything over about 30-35hz, so the extra is just headroom, which is fine also. V-sync doesn't even bother the game until you get to 60fps.
There are a few things wrong ore missing with this post. I assume you meant adaptive-vsync, as normal v-sync continues to function no matter how low your FPS are.
People see plenty higher than 30-35hz. Use this as a test: http://frames-per-second.appspot.com/
And low FPS affects latency, making games feel laggy, especially mouse driven games.