The way I see this is that it should be good while leveling. It can be good or fun when you outgear for instance heroics giving you something more you feel you can do when healing isn't the main focus anymore. In raids however I think it will be really limited to how often I will use a smite when I have time to do so, I'll rather save that mana for a heal. Perhaps if it's a raid a tier or two back who you start to outgear or early stages in a raid instance that you have really control and farm mode on.
It is in my mind situational when to use it and it's hard to tell the future but I doubt that my focus when raiding will shift from pure healing to some healing and some damage dealing.
"We have two ears and one mouth for a reason." - Buddhist belief.