One issue (or, more exactly, a Blizzard issue) with enabling more cross-realm things is that the barriers between realm communities would go down. Blizzard seems to prefer realm communities to be independent, with little intentional communication, completely segregated economy, and low player mobility. Perhaps Blizzard fear that, if players can choose any realm at any time, most players will go to realms where their chosen faction dominates and the opposition is mostly absent.

I actually think completely breaking apart the restrictions on changing realms would be a good thing; it would allow players to find their ideal realm communities without having to mortgage their houses to pay for the transfers, would allow realm communities to perhaps specialize (imagine, for example, a realm where players could go to pug 25-man raids, another famous for in-game weddings and other ceremonies, and so on), would remove most problems with empty realms (players could just leave the empty realm behind and find one they liked better), and so on.

At the very least, I don't think easily changing realms would harm the game. Rift is almost as close to an AAA-budget WoW clone as you can get (disregarding TOR), has been providing weekly free transfers for all characters for the last half a year, and this doesn't seem to have destroyed their server communities. Besides, on the faction balance front, until Blizzard actually finds a good, workable way to tackle the problem head on, I don't see how easy realm changing could harm faction balance any more.