Dancer is a great situational subjob for melee since it has the ability to heal yourself and others using TP instead of MP. As a main job I don't see it used that often so I don't know what it's useful for.

Scholar is a great subjob for mages due to the fact that it gets a JA that lets you recover MP. It also gets several other useful abilities that enhance mage spells. As a main job it is also very useful as it can switch from a healing mage to a nuking mage on the fly, and it gets some good advanced abilities that allow single target enhancing spells to be AOE based. It actually replaced WHM as the second most desired healer for most activities (RDM is still the highest), so much that SE gave WHM a huge boost in the latest update.

Jobs these days...all jobs are useful (except maybe DNC or PUP which no one seems to like). Everyone always wants BRD, and RDM is still the most desirable healing job. PLD and NIN are still desired for high end activities where tank is required. In terms of DD, SAM or WAR are probably the most wanted these days, but they're all useful really. I just depends on what you want to do.

BLU is a great job. I'm level 75 BLU and I love it. I don't even play PLD any more. BLU has such a diverse bag of tricks that they can be useful for just about anything. Leveling in the 50s is tough, that's before you get any of your big damage spells. I had a static group for that period so I managed ok. These days with level sync and FoV and other stuff it shouldn't be too hard to level any job. Just sync down to when you first get Bludgeon and BAM all of a sudden you are the best DD for your level.