There is a reason to guard a healer and I'm rather surprised that after 3 pages of discussion it hasn't be stated. If a tank can hold aggro against DPS, guarding a healer is a viable alternative if nothing more then for the 5% reduction in damage the healer would take. Granted the 5% could just be given to a dps, but frankly I'd rather my healer take less damage.
Honestly, guard whoever you want during trash pulls, it doesn't make a difference. Just don't expect that guarding someone is going to make up for a failure in the team to do a good job.
When I'm healing the first trash pull in False Emperor and the tank has opened with an area taunt (lulwhut) taken one of the droids, one of the dps is attacking that same droid, and the other dps is attacking the second droid, and nobody is bothering with the two sith or the ranged in the back, when that area taunt wears off I'm going to be killed in 10 seconds by the rest of the group. With the 5% damage reduction I'll last 10.5 seconds. Lawdy lawdy.