i dunno about that. from what i've seen, the halo 4 AR looks much better then the reach version - and people use that all the time. i can see myself definitely looking for an AR as a secondary on tighter maps.
also, dont waste a perk on firepower... primary weapons will be common place within 30 seconds of a match start. just swap out your secondary for one.
From what i saw, the AR can kill about at the same range as Reach's AR BUT it kills faster, which is the important thing. So, it is better... but ultimately mid-long range weapons will dominate majority of the map in majority of the maps. And any good player won't use the automatic weapons as MAIN weapon.
It would be a nice secondary but then one needs either Firepower (a perk i won't really use) or Dexterity (faster weapon switching, changing beetween CQB and mid-range weapon without it in a battle is not feasible, it seems) and pick it up from somewhere.
I will try to find a loadout that makes AR primary/main a feasible playstyle though, depending on map of course (won't use this loadout in Ragnarok). Say, Abandon looks like it could be a nice map for a AR-centric game though mid-range weapon will be needed too... Don't suppose the Magnum really works for this.
EDIT uh, left the second half at halfway, not thinking coherrently. Should go to sleep.