I can certainly agree that only the best captains would be trusted with the best equipment; after all, it's more valuable and therefore you'd want to give it to someone who isn't likely to wind up dead or lose it. Or, to put it another way, you want the best gear protecting the best captains because they're your true greatest resource.
However, there seriously needs to be a vendor on ESD that sells white versions of all marks of every phaser, ship shield, deflector, engine, as well as ground phaser weapons, personal shields, and armor for energy credits. It makes no sense that a captain with the energy credits can't get standard gear at ESD no matter what the level. White versions of Phaser Beam Arrays at higher than Mk VIII simply can't be bought from a vendor - anywhere.