The Han Solo Trilogy specifically talks about using the belly gun if they needed to on the Falcon and that there were interrupts to prevent the landing gear from being hit. That's a ship powered up imo though. The retractable auto fire blaster in ESB to me is a personal scale ground defense option that was self powered for ground defense.
So, here's one thing to think about. If they were to power up and fire that Quad Laser Cannon at personal-scale targets, that would be 50 points of damage per shot, and the overpenetration on those blasts would be enough to do major damage to wherever they parked their ship. And that probably wouldn't be a good thing.
You don't want to be shooting your 105mm howitzer at individual ground targets. You want to use something more suitable for that purpose, like an automated ground defense system. That AGDS might not cost any Hard Points, but I sure would make them spend the money to buy the weapons, the fire control droid, and the Mechanics skill checks to make all that fit together. Maybe that could "share" a hardpoint with Smuggling Compartments, if you wanted the AGDS to be hidden and more difficult to detect.
And I'd probably reduce the Encumbrance of cargo that they can carry by a small but non-zero amount, too.
I might even rule that you can't fire starship-class weapons at closer than Close range, much like the situation with Gunnery. And if personal-scale weapons can hit them, then you are by definition "closer" than Close range.
Of course, these all sound like House Rules to me, and it would be up to each GM to decide what they want to do and how they want to handle things.
[ Edit -- correct speling misteaks. ;-) ]
Edited by bradknowles