It is my opinion that the holster should provide one inventory slot. I think a holster would hold a bandage, fruit, can of food, matches, etc... I cannot wear it AND a vest and I cannot wear it AND a pillow jacket. There are tradeoffs involved and trading off inventory storage is part of the decision about what to wear.
In a low-inventory situation, when I am gearing up, I will completely bypass the holster unless I have a gun that needs storage. I think it should be useful for SOMETHING!