I think it's ridiculous that you can't just reset the dungeon and restart. Blizzard says that such resets would take away from the perceived difficulty of the dungeon, but I think that's just a cop-out. This game basically revolves around different ways to sink your time, and resetting a dungeon multiple times is just another way. It would only affect the people who are looking for a "perfect" run, as those who are just trying to finish the dungeon for their reward (i.e. doing their single M+10 for the week) wouldn't care about the efficiency of the run or lack of errors.
Look at Karazhan and the dungeon crawl to get Nightbane to spawn; players are MORE than willing to reset a dungeon in order to complete it within time, without regard for their lack of loot and time otherwise wasted.