I was playing a 3-minute game, my opponent had around 15 seconds to my ~50 seconds when he managed to cancel out the last pawns to make it so we each had a king and a rook. I let his time run out and he accused me of poor form. Should that have been a draw?
This is opinion based and therefore not a great fit for the Stack Exchange format, but here are my thoughts anyway.
The lower the time limit, the more acceptable it is from a sportsmanship view. In standard long time controls, this is really poor form. In 1-minute ("bullet"), anything goes, including this. The reasoning is that in these really fast forms of chess, the clock is as much a part of the game as the board is (so 15 vs 50 seconds is actually a type of winning position), whereas in slow chess the clock is more of a necessary concession to tournament organizers, not something equal games should be decided on.
I'm in doubt about 3 minute chess. It's very fast, but I'm not sure on which side of the fence it is, that'll be different from person to person.
Personally I always offer a draw when this position arises, but then I only play for fun, not for big bucks.
K + B vs K + B with bishops on opposite colors is even worse, it's hardly even possible to make a blunder in that. At least with rooks you have to watch out for skewers.
Though in long time controls, there is a slight courtesy, quick time controls is perfectly okay to win on time.