This is the only game where that's a problem - at worst, other games using this model will let you purchase something that doubles your experience point gains for a few hours, which isn't enough to get far ahead unless you're capable of the impossible (like doing several raids in that time), in which case you're probably near the level cap anyway. Furthermore, these other games being RPGs with a standard MMO equipment requirement system means you can't just buy high-end equipment and be better.
At any rate, it's certainly better than the "free to play, but if you become a paid member, you get access to the other half of the game and get to laugh at the poor losers" model of games such as RunEscape and Free Realms (which does this model in addition to the Korean model).