Ironically, I've replied to several people using physical thumbs up/down recently. This can be caused by several variations on voice not being viable at the moment:
* sore throat
* eating
* having another conversation
* context demands silence (often requires the question to be nonverbal somehow, but not e.g. if the context is "sleeping baby right next to me")
* noisy room (often asymmetric)
* target is deaf
