This is how some healers work. Regen the tank, DPS and top up with heals. Regen + Medica if required, more DPS. Or boredom, because the tank is actually good so you're just looking for something, anything, to do.
Aggro shouldnt, and rarely is, an issue if the tank is on their crowd control and/or the healer doesnt freak out. The same issue can easily appear with DPS in these situations anyway. You can interchange them, and is, again, largely down to crowd control.