Multi-cannons are perfectly fine for the class of ship on which it makes sense to fit them. Once you have a large enough ship and the hardpoints and powerplant to mount a kick-ass laser or plasma setup then of course the multi-cannons are going to seem a little underwhelming. Projectile weapons only scale up to a certain point, otherwise the US military would be fielding A-10-style single-gun tankbusters the size of AC-130s. Multi-cannons have their place but you can't possibly expect them to match the DPS of a top-tier plasma accelerator or rail gun.