Again though, most people don"t return to a hub just to sell greys, in fact that is rarely a factor in it at all as far as I am concerned. The reason I say that is because many people (I sincerely doubt I am the only one who does this...) will delete excess greys in order to make room for greens/quest items while waiting for a more opportune time to go back to town, such as: selling more valuable drops (equipment, tradeskill items), getting/turning in quests, buying new skills, repairing.
The only time this might not be the case is the early levels, in WoW 1-15 or so, but as I said there are other ways to get these effects without the annoyance of greys. Several of those ideas have been mentioned (increasing coin, increasing quest reward, increasing rarity of tradeskill drops to increase respective value, etc.).