Clarification: My issue is not with the cost of the service, or the ease of getting gold. It's that this offers a clear in-game advantage to players for real life money.
Fact is, anyone who purchases this service has access to the auction house while their character is anywhere in Azeroth. They need only to log out, do their thing on the AH, and log right back in. I have no issue with people being able to access it from work or school or anything, my issue lies with where the player's character is when he is able to use the AH.
An example I used while arguing this earlier today:
Two players are questing in the Storm Peaks, near K3. The players need one [Doohickey] to complete a quest. Neither of them has it.
Player 1 did not purchase the mobile AH. He hearths to Dalaran, takes the Ironforge portal, buys his [Doohickey], spams trade chat for a portal back to Dalaran, takes the Flight Path back to K3, and can now continue questing.
Player 2 does pay for the mobile AH. He logs out, buys the [Doohickey] on the AH, logs back in, grabs his [Doohickey] from the mailbox, and is ready to go.
This is an in-game advantage. Even more so than the Celestial Steed is.