... Biggest problem is that both camps are speaking in what ifs.
Must guard the DPS camp
They operate under the assumption that the tank is not doing his job or that the dps will overaggro the tank. This camp uses the threatreduction as their main reason why you need to guard a dps.
Must guard the Healer camp
They operate under the assumption that the DPS are not doing their job, leaving the weaker enemies bash on the healer. This camp uses the damagereduction as their main reason why you need to guard a healer
There is a third camp.
Frustrated at players that don't understand how threat works
Most of the "guard the healer" posts I've seen aren't talking about the minor damage reduction. They're generally talking about the threat reduction. "Guard the healer, he keeps pulling aggro" etc.
This is trying to fix the wrong problem. It's like, "the boat keeps filling with water, get bigger buckets!"
instead of "the boat keeps filling with water, PLUG THE LEAK"
Now, if guarding the healer actually WERE a solution for a bad pug, that would be fine. But it isn't. The only reason the healer is getting hammered is that the other players aren't attacking the mob. Guard won't do anything. Heck, guarding the dps won't do anything EITHER.
To me, it's not a matter of "guard the healer" or "guard the dps". It's a matter of, "if your healer is getting ******, asking for a guard isn't going to fix the *********** problem!"