Well, I basically agree that a tank should have some lighter secondary armament to combat infantry. Even if don't need to conserve ammunition, we might not always want to use the big sledgehammer with the high signature and massive collateral damage.
A "laser fifty cal" (or two) able to defeat battle dress would be ideal.
MT has rapid fire Pulse Lasers, but they are not all that cheap and less capable than plasma or fusion guns. Light auto-cannon offer far superior rate of fire.