Respawning monsters work better in games where the battles don't take long. I don't really feel like fighting the same three groups of enemies three times because I forgot to bring some important quest item.
That's why I'm really suggesting to do it in an OPTIONAL area of the game. For the whole game no, I agree, I like it the way it is. But once you've done everything there is to do then it would be nice to have an area you can go to with respawning monsters. They don't even have to be worth XP for all I care, I just want more stuff to kill without restarting the game.