For the most part, the spent casings are ejected away and would disappear rather quickly so I won't worry that much about the casings affecting stealth.
SkullLeaderVF-X, for more information, I would suggest reading up on the Heckler & Koch G11 regarding more information about caseless ammo and its modern applications (keep in mind, guns, like muskets, were caseless back in the day ).
One other issue is energy consumption. Something that the VF-0's entries in the Chronicle mention is while Battroid-mode uses only a small bit of power, while energy converting armor while in Battroid consumes a a majority of the rest of the energy. So any beam weapon would have to compete with other systems for power.
But, the one overriding issue with beam weapons as primary weapons is costs.