as quoted from the Tome of Knowledge:
"ALL pets can use armor on ALL slots
Pets have a fixed delay
pets have a set amount of attacks so making it wield two weapons is only good for increasing its amount of procs."
So in the case of a Beastlord if you have the procing buffs on your pet, say Spirit of Summer, handing it weapons will increase the amount of times the proc goes off, which increases the damage the pet does.