Even if server load isn't a problem, what if you got put into a group that either couldn't practically finish the dungeon or would take 5 hours to finish the dungeon? Dungeons are locked to 90-120 minutes usually and even DF groups complete them well under that time because a standard has been set by the timer. Remove the timer and you'll get slightly worse players and longer completion times. It's a trade off between having ridiculously long runs occasionally vs. reasonable length runs with some occasional fails.