If I'm in a Catapult B, I'm not going to waste precious MRM ammo on a BA. Unless it's an emergency and I absolutely have to kill the BA instantly, the Mguns work just as well.
The Puma C's CERMBLs do work well on BA, but generate a lot of heat and take time to recharge. I can fire Mguns nonstop and reserve the lasers for heavier targets.
The same logic works for just about any asset. Ballistic or missile weapons have limited ammo and energy weapons generate excess heat, both of which you might not want to waste on BA when you have bigger fish to fry. Larger weapons of any kind also often have long reload times, which makes missing with them especially painful.
If you're only going to fight 'Mechs with them, by all means use SBLs. Mguns aren't designed for that. They're a light support weapon to defend against light armor. They're even useful for harassing aircraft, with a high damage multiplier and no hard maximum range.