Just thinking about a tank who also heals himself while doing his normal attacks could be generating more threat than a tank who can not self heal. Doing high amounts of damgae (threat) + being in correct stance/presence/form (more threat) + healing (a little threat) = a lot of threat.
The idea of self healing intrested me from a healers POV. In cata they said we will have to worry about mana more, with a tank who can heal himself/herself a bit maybe my mana concerns will be less.
Just a side idea about how to make healing "more fun," Boost its threat. Then if a tank isn't geared enough (aka a stam stacking mana sponge) instead of having to worry about running out of mana before dps can kill the boss, you have to worry about pulling threat from the tank because you are having to heal him/her so much.
That way healers would have to decide if they want to take talents that makes their threat less like http://www.wowhead.com/spell=14785 and http://www.wowhead.com/spell=17120 and http://www.wowhead.com/spell=29191 for tanks who are still getting geared, or just go for thoughtput talents for fear of pulling agro.