Keep in mind that the "titles" aren't titles, or even classes. They are character roles that the design team is using to determine how a particular class will fit in with a group. Thus a controller could be a cleric, wizard, ranger, paladin, fighter, rogue, etc. so long as he performs the "controller" role, while the "striker" could be any of those provided it performs the "striker" role. I'm sure that the rogue and the ranger will be better "strikers" than a paladin who presumably makes a better defender or leader, but that doesn't mean that those roles can't be exchanged between party members.