Here is what I can tell you when it comes to my healing...
When I use a healing trance on someone if it crits all 4 ticks (which it does a lot) thats an 8k heal to the tank. If I follow that up with a deliverance and it crits, thats an additional 6.5-7k heal. Thats about 15k in heals with just those two heals. If I throw down an AoE heal that is potentially another 5-6k healing.
So we are talking 21k healing just to the tank. I can do that in about 15 seconds. That means that of all the additional adds around us it will take according to the chart 11k damage by one of the DPS to pull ONE add off me. What if there are three adds in the area or more? Like in the beginning of Maelstrom Prison?
With the guard on me it will reduce that threat by 50%. VERY necessary if the tank whats to scoop those adds up with an AoE taunt.
You're forgetting 2 things.
Heal threat reduction talent, brings that down to 8k. Any DPS or tank that can't do 8k threat in 15 seconds is terrible.
Heal agro is split amongst all targets in combat. So with 4 targets that is now 2k on each target. Which is 1k DMG for a tank.
If you're getting agro from that its literally mobs that no one has sneezed loudly at, and that won't change whether you have guard or not.