[urtie]

The problem with space sensing interfaces is one of intentionality. The best example of how this is hard that I can think of is that of a passive IR sensor wired to trigger opening a door. If that door is parallel to a hallway, the door will probably be opened a lot by people that have no intention whatsoever of going through the door.

It is that particular problem that is well captured by buttons: by decreasing the actual sensing area you increase the intentionality of the measurement. Furthermore, requiring specific directional movement increases it further.

Another example: the capacitive sensing 'buttons' on my washing machines can be accidentally activated by touching with the side of the arm, which can occur for instance when I'm loading detergent. That accidental activation is harder to achieve and easier to detect before accidental activation for physical buttons that have some travel.