The problem is that potions are good seller, especially among casual crowd.
IMO, the cooldown on pots would make them less useful, while still keeping sales. Vast majority of DDO Store SP Pot sales comes from casual players - players that spend a lot of time doing quests, don't care about optimal build and gear, and that are willing to pay money to make easy (for hardcore crowd) quests even easier, to make them completable on their skill level. Only time I've ever seen such pot was when it was being linked by guildie I was helping to do New Invasion (it was pre-u9, so at the end I was forced to go down into pit alone and Necro Blast the boss - was very annoying).
So, assigning timer would nerf pots for hardcore players that buy them with pp and hoard stacks of 100s of them; while still making them desired for casual players who buy them from DDO store.
EDIT: Not that I care either way, though... if somebody wants to drink a bunch of pots, big deal... not my problem.