Different hardpoints is a good answer, mechs like K2 Catapult and the Cataphract with all those ACs have different capabilities than other mechs in their class that recommend them. I'm not sure this plays out too much in assaults so far, as no assault has double ballistics yet. Maybe my point of view is particular to my play style, I prefer LRMs at range and enough medium lasers to not overload my heat dissipation, you can get that from any of the 3 current assaults right now.
I flit among three Stalkers and an Atlas. I started with the STK-3F, because I was actually getting decent in the STK-3F trial mech, then I got a -5M for the centerline energy hardpoint, and a -5S to get a trio of Stalkers to unlock all of the advancements. The -3F now runs with 4PPC's and 2ML, the -5M runs with the ERLL it arrives with, stock, 4MPL's, 4SRM6's and an SRM4 (Scattershot). The -5s runs with a 300XLFE, 2MPL, an SRM4 and 4ERPPC's (Sleepy*). I have an AS7-D-DC with the AC/20, 2LPL's, 2SRM6's, and an SRM2 (Li'l Edward). The AS7-D-DC has two ballistic hardpoints, but both in the same side torso. I may take out the AC/20, downgrade the LPL's to MPL's and mount 2UAC/5's.
My -3F and Sleepy are hang back and snipe mechs, Scattershot is a splatcat wannabee (if you meet Scattershot in a cave, wait for me to shut down, and then run away; unless, you can kill me before my restart). Li'l Edward can hit with the LPL's for reduced damage to 600m, but really is an in-your-face guy. Li'l Edward has an ECM and the three Stalkers each have a BAP
Since my last post, I have learned something annoying about the Atlas: It does not shoot well down hill, nor is it able to fire over a ridge line without exposing itself all but completely (cockpit in head, weapons at waist level). Stalker arm weaponry is at cockpit level, so anything you can see over, you can fire over, and you can lower the nose to fire down. Catapults are even better, as with a bit of cleverness, you might be able to shoot over ridge with only the arms exposed (would need to try this out).
* Sleepy was a money pit. To cram in enough double heat sinks, I had to replace the 255XLFE with a 300XLFE, so the engine, heat sink upgrade and ERPPC's added 8.8 million c-bills to a mech that was not cheap to begin with.