While MMO instinct is to do different things on different characters, realistically you are much better off sticking to a single character. You aren't capped in terms of the total level you can achieve with all of your jobs - in other words, you can reach 50 with all battle classes, crafters, and gatherers. And because of cross-classing skills and much gear being useable by multiple classes, you will even be able to benefit from having others levelled.

Plus, levelling up a battle class that isn't your highest level one gets a +50% EXP boost, so...