Should I continue leveling ARC?

[Lium]

I'm currently leveling a THM and have it at level 25. I want to get to 30 to unlock Black Mage. But I'm wondering once I do that, should I go back and finish my archer? I feel like I'll be devoting all my time BLM.

[Ehr]

Assuming the forum is correct, you have ARC at 18 which will let you unlock BLM. However, at some point I recommend getting ARC (BRD) to 34 for Quelling Strikes so you have that to cross class onto your BLM. It reduces the amount of enmity you generate. But otherwise, you don't NEED to level ARC/BRD completely.

[Sagittarian]

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.