Guard reduces damage taken by 5% and threat generated by 25%. A good healer will, in MOST circumstances, be outside of the 15 meter range on guard. In addition, at range, it takes 130% threat to pull off of a tank. This means that the healer will only pull threat on mobs that just aren't being hit. It is the tank & dps's job to hit the mobs. Guard on the healer is not an excuse to use them as a tank. Overall, putting guard on the healer in PvE is generally a worse option than putting it on a dps who is likely to pull threat or "tank" smaller mobs.
See, that's why I like this guy. When I've reached IDGAF territory and can't be bothered explaining things anymore, he's got me covered