For the added extra weight of a BUG, its much more economical to carry a spare magazine or 2.
Besides I figure if you are in a situation where you empty your primary weapon, reload, empty it again, and have to switch to your BUG: You are in deep, deep, Doo-Doo.
That BUG is not going to make that much of a difference then. Unless its an Vulcan 20mm Cannon with 10,000 rounds of ammo.