I don't think they have said that each class can only open one job in the long haul. I think they have said that initially a class can only open one job. To me that implied that maybe at first a job like Lancer might be limited to only opening Dragoon for example. But down the road they may add, say Chocobo Knight as a Lancer job. And at that point a Lancer could possibly have the choice to open either Dragoon or Chocobo Knight or may could do the quests for both and have both choices.

The reason why this would be beneficial is to have more variety based on available players and needs for a raid. Take Gladiator for example. If Glad opened jobs like Paladin and opened a damage class, let's just call it Knight, then player using Glad could switch to either Paladin (tank) or Knight (fighter) depending on the needs of the raid.

Who knows though. All guesses at this point. Hope we see multiple choices for jobs though.