Tactically, why would anyone stay in cover if they had a melee weapon?
It seems to me that the tactics are being dictated by the nature of the melee weapon wielders weapon, and not by the nature of the gunslingers weapon.
If both combatants are wielding firearms, is there an existing reason why they wouldn't attempt to find cover and attack from range? If both combatants have effective missile weapons, is there some particular rule exclusive to firearms that makes it desirable to close the range? I've not got a lot of experience here, but it seems to me that in prd if you have a bow the optimal responce to a firearm is stay more than one range increment away (thereby negating the firearms one major advantage, touch attack at close range). If you can't stay beyond one range increment, the advantage of cover especially at high level is going to be outweighed by the firearms touch attack regardless of any other changes. So you must either close to melee or else move back. You have no other tactical choice.
If at least one combatant is wielding a melee weapon, won't they attempt to close by the most expedient method? What advantage do they have to stalling, behind cover or not?