It sounds like you need a better understanding of healer threat mechanics.
Not only is healer threat 50% of healing done, there is an additional 15% threat reduction from the healer skill tree (e.g. Foresight for Sages), and then the threat is divided among the mobs present.
Let's say your healer is doing 1500 hps and there are 4 mobs present. 1500 hps - 50% - 15% / 4 mobs = ~160 threat per second per mob. If you guard the healer that goes down to 120 tps. How is this helping?
If the healer is pulling aggro with 160 tps, it's because nobody is attacking that mob. Reducing threat down to 120 tps is going to do exactly jack diddly squat.
Exactly. A guard on a healer is a wasted guard since it does absolutely nothing.