It depends on the game, whether the purchases will make it unfair or not. Some people just don't have the time to play as much, due to their job, so they use the money from work to get a boost in their favorite game, to still play at the level of who ever it is they play with. The other players are normally ones with no life or job and only play the bloody game anyway, so is that unfair? It's just one of those things that actually bring more balance in the game to the "normal" player.
Still F'ed up that they force people in jail to farm though.