If you go over 30mph you'll probably miss them. For instance, on rolling hills I like to build up speed at the end of one downhill to get some momentum to help get up the next hill. It depends on your route, but in some cases you can really make use of high gears even when you're not in a high performance mode.

Off road high gears aren't used much, so mountain bikes often don't have them, but on pavement it's a different story.