I personally have no real reason to faction change but i know a few people who might.
I know that some of our guild already have both alliance and horde characters on the same server, either on one account or multiple, gives them a chance to play some of their characters more i suppose if some of their alliance ones are "shelved" so to speak.
Also i know my younger brother has alot of alliance characters, and has expressed an interest in potentially moving one character to SSL and swapping it to horde to join the guild.
I dont really see the faction change being used extensively by many people, it just opens up more options for players who spend alot of time talking to players on other servers and factions.
To be honest what id be interested to see is Cross Continental transfers. I know alot of people who play on US servers, and given the chance id enjoy playing along side them, but i doubt thats an option we will see for a long time, if ever.