It does waste time for both sides,but it's worth the practise. the winning side can learn how to keep the pressure, and the losing side can practise defencing the best way they can.
besides, unless it's a tornament, it's best not to resign. in a tornament, you have to save your energy and time.
Every game is a lesson, no matter won or lost.
Resign = Definite loss.
Play on = Likely loss.
Now, which option do you want to choose in order to maintain a good rating and have more fun? As a low rated player, I can say I have been saved by stalemate (and made stalemate traps) so many times. Sure, it'd be lovely for everyone to resign against me in a losing position, because I still win. That's exactly why the opponent won't resign.