Do what you feel like playing.

As mentioned, you'll want to level archer enough to get quelling strikes - it lets you pump out more damage with less worry about stealing aggro. Other than that it's up to you - there is no benefit beyond the cross-class skills and they are going away in Stormblood. Many people get one job to 60 and then go back and fill in others for fun. If you enjoy BLM you may also like SMN or the upcoming RDM.