Smart heals aren't always the answer when healing. Sometimes you don't need to heal the person with lowest health first :-)
Not always, sure. But with a spell as small as Heal (or Smite), you're passing the time until you can't get away with just that little bit of HPS anymore. You're never going to be watching your tank get an unholy beatdown and be crossing your fingers that Atonement picks her to heal.
Don't forget the tools Discipline Priests have to spot heal and/or guide the Atonement. If you've got the tank squared away, you can Shield her and know that your Smites will be doing some spot healing on raid. If there's light or incidental AOE damage going around, your Mending can do some clean-up to equalize health values while your Smite heals are more likely to land on the tank, who's taking more consistent (and probably heavier) damage. If you do fall behind on tank healing for a bit, or just need to pick someone up, hit Penance.
People seem to be making an unconscious rule that if you're Smiting, you shouldn't be doing anything else, or that if you're healing without first cashing in your Evangelism stacks for Archangel, then you're doing it wrong. Evangelism has a relatively long duration. You have the freedom to do a lot of other things in those 15 seconds. =)
And again, Smite's HPS is (intended to be) better than Heal's. Evangelism reduces Penance's mana cost. Archangel gives you a running start when you have to return to healing full-time. The "I can't control where it goes" thing is debatable in itself, because it's a smart heal that is performing healing you'd have to do at some point anyway. But the developers have given the spec a lot of icing on the cake, just in case.