Samurai is going to be a DPS class. I can't find where they discussed it directly right now, but it was in a live letter or an interview at one point.

As for what happens if they release it along side Red Mage. My money says Red Mage will be our next tanking class. I can't figure a better way to represent their theme of melee weapons, black magic and white magic. Making them a DPS or a healer will require they minimize one side of the magic and likely eliminate and melee action they would take part in. As a tank we get melee activity and maybe some weapon skils, Black magic could be ranged threat generators, DOTs, and maybe something like an energy shield that damages attackers hitting them. Meanwhile White magic can be represented in defensive buffs and self heals. Perhaps the En spells (Enfire, Enthunder, Etc...) can be stances.