I mean I've seen instances where there's been two 72 geared sents/gunslingers and the tank goes ahead and chucks the guard on some lower geared healer.
Is there any reason why this still happens? Bad advice become force of habit perhaps?
I have a Guardian Tank, and lately, because of the spiky damage a Shadow Tank takes, I find myself tossing Guard on them(Ops only and in certain instances) and pulling the aggro of whatever they were pulling just to save the Shadow.
Other than that, I usually don't guard anyone, because I can hold aggro. That's the point of a tank, work your rotations and cool-downs and you shouldn't have a problem.....ever.