Hm, i get your point, but isn´t the weapon used mainly depending on the tanks purpose? Can you simply say they never will utilize beamweapons without knowing what kind of weapons will be developed in the future (or in the writers imagination). Personaly, i kind of like the idea of a tank with disruptor beam cannons, like the ones on the Klingon Bird of Prey, vaporizing any matter the hit!
But i agree again with you that in the near future, beam weapons are out of question, simply cause we´re not able to bring enough energy in a tank to get the same result as a smoothbore.