There is a reason to guard a healer and I'm rather surprised that after 3 pages of discussion it hasn't be stated. If a tank can hold aggro against DPS, guarding a healer is a viable alternative if nothing more then for the 5% reduction in damage the healer would take. Granted the 5% could just be given to a dps, but frankly I'd rather my healer take less damage.
Yep, that is 100% correct. However, because dps are very overpowered in this game with respect to tanks, you almost always will have to guard the dps for the first 20 or so secs of a fight w/ similar gear/skill of course. Because tanking stats add absolutely nothing to aggro generation, the higher tier gear the group has, the harder it gets to keep aggro. You do get some extra mainstat, but the secondary stats are all tanking stats, while the dps gets secondary stats that add to dps which adds to aggro.