I would say is because of giving more people reason to grind? Some people have motivation or incentive to grind if switch GC and aim to stay in the GC to play with friends so they will most of time do not mind re-level.

But maybe this idea form because was implemented when many player still PVP, which also mean right now and until 2.4, will remain to look 'dead' until 2.4 patch gives new PVP incentive?