Having personal module storage (at a price) would remove one of the more nonsensical issues you get when upgrading your ship modules.
E.g. upgrading the cargo racks in a new Lakon 6, two size 5 compartments with size 4 racks (16 tons), two size 4 compartments with size 3 racks in (8 tons).
You have to make sure the station sells both types of cargo rack you want to buy. Presumably the work shy, bureaucratic technicians won't let you slip them a few hundred credits just to move your size 4 racks into your size 4 compartments leaving you able to just purchase the two size 5 racks you were after?
"No, no, no mate, sorry, union rules isn't it. We don't supply that size rack here so it can't be done, like I said, it's the union rules. Don't even have the right sized hydro-spanner."
"Try Aulin or somefink."
Plus occasionally it would be nice to re-arrange the order you have some of your utility modules/weapons fixed in. Maybe it's just me but I like to have my energy weapons and kinetic weapons grouped together.
Do I need a life? I do don't I, I need a life.
But I've just bought an Asp....