They are only situationally useful. On single target fights there a bit of a waste, if the tank looses aggro its going to be to DPS not the healer, and unless its a very poor tank then your unlikely to pull aggro off groups of mobs too.
The most dangerous part is when adds spawn partway through the fight. The only time I use strings is when I'm going to start using big heals just as the tanks are trying to get everything under control.
But I rarely PUG so perhaps I have a skewed view of how easy it is to pull aggro off a tank through healing.
“All we have to decide is what to do with the time that is given us.”