Read this post, it covers the same issue: http://forum.square-enix.com/ffxiv/t...er-vs-Multiple

It's foolish to not use one character and level all your desired classes on it. You can easily level your first class by doing the story quests, FATEs, levels, hests, dungeons, hunting logs, grinding, ect. When leveling a lower class you get an exp bonus and you can still do everything I mentioned except the story quests; instead, you should do the side quests. It's really not a hindrance, and the exp bonus more than makes up for not being able to do the story quests.

Read the replies on the thread I linked; I replied on the third page with a more detailed version of what I've just written above.