The PPC seems like a far superior weapon to the AC/2:
PPC = 10 damage out to 18 hexes, for 7 tons, 10 heat, and 3 critical slots.
AC/2 = a mere 2 damage out to 24 hexes, for 6 tons plus 1 for ammo, 1 heat, and 2 critical slots (one of those for ammo)
You can only put 1 AC/2 into a chassis for the same tonnage as a PPC, and it only does 1/5th the damage.
On the other hand, the PPC requires 10 heatsinks versus 1 for the AC/2, and if you have to add the heatsinks (such as on an ICE powered vehicle or chassis with other heat-building weapons), that jumps up to 17 tons to actually USE the weapon effectively. Then you figure the power amplifiers you need to convert power for a PPC on an ICE vehicle, and that adds a few more tons, to the point where you could get 3 AC/2s instead, or in many cases simply can't put the PPC on the vehicle at all without resorting to a fusion engine. If the target is 19-24 hexes away, the PPC is useless. At high to-hit numbers against ranged targets (particularly VTOLs and hovertanks), the AC/2 may be in a shorter range band, increasing its odds of hitting the target at all, and a 2 point shot that hits and causes a motive critical is far more effective than a 10 point shot that misses.
On a 'Mech, it's a niche weapon, typically only useful against ASFs, vehicles, infantry in the open, or for slowly degrading fixed fortifications. It's also a viable SECOND ranged weapon for a 'Mech or fusion-powered vehicle that's already making use of its 10 "free" heatsinks (I'd gladly swap an AC/5 on a Marauder for an AC/2 and some extra heatsinks). On an ICE vehicle, it's a very viable weapon alternative in general, although of only limited utility against BattleMechs.
In a company-plus sized 3025 force, I'd prefer to have at least one AC/2 available. I'd much rather have a LOT of LRMs or PPCs than a LOT of AC/2s, though. The Marik designs post-Jihad have the same problem with the LGR as I would want to avoid with the AC/2: far too widespread use of a relatively specialized weapon, yet the LGR is a "good" weapon to have, at least in small numbers. If you can make use of that specialty, it's valuable. If you're using it as a general-purpose weapon, it's simply not tonnage-efficient for the damage it delivers.