Blizzard is going to knock out two birds with one stone during the next major content patch (3.2). Not only will we be able to earn real XP during PvP, those twinks not wanting to inadvertently level up can turn experience gains off. As Nethaera says in the announcement:

War is an experience that few would choose to have in their lifetimes, and yet, in Azeroth, it is the way of the world as the Horde and Alliance continue to struggle against each other even as more imminent dangers split their focus. It is this experience that forges the novice into the hardened veteran and drives them onward to fame and glory. In Call of the Crusade, the opportunity to gain experience through war will become a reality.

With the upcoming content patch, players will be able to gain experience within the Battlegrounds for completing objectives and actions that yield honor in them. These objectives include (but are not limited to) capturing flags in Warsong Gulch, gaining resources in Eye of the Storm or Arathi Basin, or taking objectives in Alterac Valley. Player level will also affect the amount of experience that can be gained within the battleground.

For those that would like to opt out of gaining experience while in the battleground, you will be able to visit Behsten in Stormwind or Slahtz in Orgrimmar – both located near the battlemasters in either city – and turn off all experience accumulation for the cost of 10 gold. Players with experience gains turned off who compete in Battlegrounds will face off only against other players with experience gains turned off. Behsten and Slahtz can reinstate experience gains for players, for a 10 gold fee of course. Any experience that would’ve been accumulated if experience gains were not turned off cannot be recovered.

She later goes on the clarify...

It will turn off all experience gains whether in the Battleground or out of it. This can have its uses for those that perhaps just want to level to a certain level and stop.

Some possible other benefits with this (caveat being things could change in how it functions during testing):

Players who want to create "Old World" guilds and do "Classic raiding" can level to 60 (for example) then turn off Experience gains.
Players who want to level with a friend but are a faster leveler, could turn of XP gains for a bit so they could still play but not get too far ahead of their friend.
Players who like to Role-play at a certain level, can turn off the XP gains and not worry about getting outside the bounds of their character.

You can read the official forum here.