Since cross-realm grouping is possible, we know the next step - something like oQueue that allows you to put together a group of 40 players and go destroy Ashram on an already imbalanced server. If a certain server is already heavily skewed towards the Alliance, putting together a 40 player group (since Ashram exists in the world and not in a raid instance) and just destroying any hapless Horde you come across, or vice versa. Even if you don't pick a server with a faction imbalance, it's still feasible that a big raid group could end up owning Ashram for an extended period of time, and using players that aren't even on the server.
Decisions in the game's design are always made between these two poles - between the ease of abuse, and the benefit it brings to individual players. Sometimes, even with the best of intentions, things don't work out like we'd hope. Reforging, as an example, falls into the 'possible player imbalanced use trumps player convenience' category.
When reforging was introduced, it's intended use (as presented to us) was that it would allow gear that was intended for, say, tanking or healing to be tweaked so it could be used for DPS, or vice versa. Players being the rationalists they are, the full potential of the system was exploited, and soon people were using addons and calculation websites to come up with the absolutely optimal reforging for all gear. No longer were people reforging tank pants so they could use them for DPS as a minor upgrade until something better dropped. Now, even a full set of healing gear would be reforged to provide the absolutely maximum benefit for the class using it. If your class hated critical strike and loved haste... then as much critical strike was going to become haste as the system allowed.
This comparison wasn't meant to imply that Ashram is simply not going to work with cross-realm grouping. It's merely an example of how any and all such edge cases must be considered. Sometimes, they really are edge cases. An example of this would be the bonus roll system. It's possible to game it, for certain classes and specs, but the gains are minor at best (a fury warrior rolling under arms, so as to take 1h weapons off of his table) and it's not breaking anything. Yes, you could stockpile your coins and wait until you have 10 bonus rolls ready to go, burn three, do another turn in, and then get ten more. So what? It averages about the same. It doesn't end up dominating how bonus rolls are used the way all players started using reforging to compensate for stats they didn't want.
Another example would be the original ladder system put in place with Battlegrounds when WoW first introduced that feature. No one foresaw that players would break the ToS and share account information in order to progress up the ladder faster by playing characters in shifts. Eventually, it became the only way one could expect to climb the ladder, and so, the system was scrapped and a new one put in its place. Sometimes the balance is achieved and player interest is served. Sometimes it isn't. The system introduced in Burning Crusade for Badges of Justice, which led to the Justice/Valor system we have today, worked for nearly seven years. If it seems to be ultimately to be phased out to a degree in Warlords, that doesn't change the fact that for many years it served the player interest (letting players compensate for poor loot rolls and get alts geared up faster) and didn't necessarily imbalance the game. Many adjustments were made over the years to compensate for edge cases (some players would stay up and grind Badges/Point until they had every piece of gear they could buy, even when it wasn't necessary - so caps were introduced) and now, more seem to be coming.
Ashram allowing cross-realm groups may end up being similarly adjusted - perhaps a cap of how many people can be groups there at once, or some kind of zone-wide buff to the team with the least players, or perhaps something totally outside my experience as a player. The point is, the game has always had this tension between these forces, and it's fascinating to see how Ashram highlights that discordance.