You keep saying that, and yet I pull aggro - and don't tell me I'm overhealing, because I'm not.
I'd like to see definitive proof that healing threat is 50% base before being divided among all mobs. I've also seen people disagree on whether it's actually divided among all mobs or not. ::shrugs::
The guard makes a difference. I've SEEN it make a difference.
The 50% base with the 15% skilled reduction can easily be seen by looking at combat logs. Just check out any raid on torparse.
Nobody is trying to say that guard will have no effect. It just has a minimal effect on healing threat, as it is so low already. Frankly on trash pulls you can guard the healer, guard the dps or guard NOBODY and any decent group will get through it. It's just not as important as kill order, CCs and correct use of taunts. If a group is having trouble and thinks that guarding the healer will solve something, that's the problem.
Study the combat logs. If you have a raid where you (as the healer) pulled, you can upload your log to torparse and check it out.