First, let's look at some stats:
Damage: 3 at 90/180/270m, 2 at 150/300/450m, and 1 at 270/540/810m
Let's just take the range profile of the "1" damage and use it as the damage falloff profile to get:
Adjusted Damage: 3/2/1 at 270/540/810m
Weight/Crit/Heat: 1.5/2/2 for ATM3, 3.5/3/4 for ATM6, 5/4/6 for ATM9, and 7/5/8 for ATM12
As you can see, ATMs are not balanced around long range (1t of ammo would be 60 missiles) so to make ATMs can still do something at that range, they need to behave like SSRMs with their "bone-homing" targeting system. With that in mind, you can hope to leg a Light with its leg armor open even at medium-long range. This does make Lights more vulnerable.
ATMs are in fact, short to mid range weapon when you look at the stats. Now you might be wondering what will happen at 270m or below if someone pack multiple ATM12. A ~70kph Scorch (Marauder IIC), for example, can pack 4x ATM12 with 6t ammo, 17 DHS, and a couple of ERMLs. The ATMs would alpha 144 bone-seeking damage at 270m or below.
I don't remember the probability for each bone to be hit but this could 1-2 shot a Light at 270m or below although the alpha heat would be prohibitive because of GH. Note that the above mentioned Scorch is basically too restricted by the heavy ATMs.
ATMs could have LRM's targeting system but they would be completely useless at long range though it could balance their massive (and hot) short range alpha potential.
TL;DR:
- ATMs target bones like SSRMs
- 2.7/1.8/0.9 damage at 270/540/810m
- 160m/s (LRM speed)
- Susceptible to AMS
- Will cause much salt to be shed because people will hate it that they can't run away from the missiles
- Crippled Lights everywhere, are still not that great for heavier 'Mechs unless they have opened sections with juicy weapons (hello Gauss 'Mechs!)