Definitely jumping for exploration. Combat-wise, it would need to be worth it mechanics-wise. Like some people were saying, avoiding stuff like earthquakes would be sweet if there are no latency issues. Otherwise, the only real combat use I'd like to see would be for moving around/positioning around an arena in order to avoid certain attacks. For example, a boss could be fought in an arena that fills/drains with poisonous water, so you have to jump to higher levels to stay out of it.

Regarding what button to use on the gamepad, looking at the 2.0 screenshots we are getting an active mode toggle on the action bar so they could just get rid of that button on the gamepad.