If you really want, try something really crazy and make swordsinger dualclass in robes, or a safe option would be phantom summoner. You can change dualclass later anyway and keep the levels.
So, having a dualclass and subclasses gives you the option to gain passive skills which will boost magic attack, defense, cast speed, etc, depending on what you choose. I would develop your dualclass right away because you could be doing some quests as the dualclass and switching to healer to complete and take xp rewards. That will be less painful than playing healer the whole time.