There is no point to ever guard a healer in PvE if the tank and the dps any clue on what they should be doing.
When I pug I never just assume my DPS know what they're doing. And I don't feel like spending some time grilling them about kill order. So I tend to just put Guard on the healer because I assume I'm the only one with any sense. I'm often right, though I don't have any pride about switching Guard if I get that magical pug DPS who is both well-geared AND well-played.
My Mara is an aggro-ripping machine and I love getting Guard on him, but it happens pretty rarely! But I don't feel like bossing the tank around and potentially starting an argument with someone who is either too dumb or too proud to think I need it. The worst I ever have to do is manually type 'taunt please' and then smile smugly to myself cuz I'm sooooo smart!
But I could also forget Guard and we wouldn't wipe. Wouldn't even come vaguely close to wiping. Probably wouldn't even make a difference. This probably has more to do with the fact that I have target-of-target on and I can tell when I lose aggro. If your tank is failing, it's probably not because he misplaced Guard.