You can do that, but many boards are really designed to sit on top of the card. In a robot that walks you don't have that much space and you want to mount ever

You can do that, but many boards are really designed to sit on top of the card. In a robot that walks you don't have that much space and you want to mount everything as firmly as possible to avoid a bad connection from repeated vibrations. It is possible but so far it hasn't seem worth the hassle.