Originally Posted by ShannonM

Just because you can doesn't mean that you should.

For "shifting while standing" the "why you shouldn't do that" case is this:

You try for a way-too-late desperation bail-out shift to your lowest gear, standing on the pedals, heaving on the bars, at full honk, and in time with your stroke. And then your rear derailleur blows into three pieces, and then your rack your 'nads on the top tube, and then you crash.

(This is why I stopped using GripShift on mountain bikes, despite my intense dislike for under-the-bar push-with-your-thumb MTB shifters. And thumb throttles on ATVs.)

--Shannon