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.
I'm fairly sure that guard only gives that 5% damage reduction in pvp.
Edit: Nevermind, rechecked the ability. 5% damage reduction is in pve as well.