I am playing as AP and I feel I have to add something to this discussion as well. I have played around a bit as Pyro as well and when running hard modes I find that I do more constant damage as AP than as a Pyro. The damage as Pyro is more spiky, if you get a good proc streak the damage output is awesome, but if you dont get a good proc streak then I feel that the DPS output is lower then the AP.
I can only concur about the lack of utility at times, as an AP I bring short CD on interrupts and the ability to pull mobs closer to the tank. We do not have any aggro dumps, we do not have any CC's and we do not have any pushback.
What I feel I am bringing as an AP is a good steady stream of damage, alot centered around the flame thrower with talent stacked.
If I am not guarded the tank can not keep aggro for long, if I am guarded I still do pull aggro on occasion. Though if one aint too over zealous it is not a major issue. Main problems come when death from above and flame throwers are used in sequence a bit too soon in a fight with several mobs.
Though you do not see very high numbers when playing AP, but it is a steady stream of numbers. examples would be immolate crits for around 3200, flame thrower for around 2400 / tick, rail shot for around 2700 and flame burst for around 2300. More if popping relics etc.
The main reason why I am running as AP is the heat situation. You have to be alot more focused on your heat bar and weave in alot more rapid shots than if you run as AP. Which makes your damage go alot more up and down as Pyro. Some might call it lack of attention and they might be right about that as well
I would say that I find it a fun class to play as a DPS, and Pyro or AP are both viable for HM's. And that you should go with the style you find the most satisfying and fun to play, since thats where you will perform the best and do more damage.