Though tactically questionable (as others have mentioned), there's nothing wrong with trying to explore different strategies. It can get stale if every combat involves two armies running together, meeting, and duking it out on a line for a few rounds. If your players are sneaking around to look for an advantage, maybe you can tweak things so it doesn't take as long, but they still have the option. There are a few ways this could be done.
Firstly, if the area is built to allow sneaking around to flank in a viable way (a balcony that runs the length of the room on one side, a small side corridor that links back up at the back, etc), then the PCs can get around while only missing one, maybe two rounds.
Secondly, the area could include a highly defensible location, where the tank could get a bonus to AC (giving him more staying power). This might help him hold on for a few extra rounds, while the monsters battle against him. Ideally, it'd be something temporary; one monster could be fighting him, while the others tear the defenses apart to get at him.