Old Soviet saying - "Infantry is the queen of battle, but Artillery is the king of war. And we know what the king does to the queen."
Same thing here. If you're planning to deploy massive fieldworks, manned with these, expect artillery. It's the "weapon without answer" to this solution.
It all comes down to balance. You can probably get similar damage output from an SRM-3 launcher - up to 6 points damage at 9 hex range - as with the medium laser, and have mobility, more armour, or fancy things like stealth armour, and have something which isn't helpful outside one specific circumstance.
In the 'Mech design paradigm, there's a failed one - "speed is armour." However, most people agree after trying it, you only have to connect once with the speedy unit to cripple it, on the whole. There's a reason armour is armour :)