Although it is possible to make a CMOS or even a single open-drain NMOS so that it withstands continouos short to power supply/ground, I don't think it is the common case with '51s. CMOS with both transistors open is an another issue.

The point is, if you press multiple keys in such a configuration, the current increases and may get beyond what the open drain driver withstands. This can happen even with the pullups, if there are enough keys to press. Of course, this is not too likely on a 4x4 keypad, but there are bigger keyboards/switch matrices around. Not necessarily resulting in pooh, it might make also shhhh :-). Been there, unfortunately.

Serial diodes reduce problems, if likely.

So, as with any other electronic circuit, it is good to make a few calculations...

Jan Waclawek