I'm not a BLM but I'll try to give my opinion on MP gear.
The only thing MP would have any use for is to boost your MP up so you can get off a few free nukes, and put it away until you rest to full again. MP is only as good as money, once you spend it its gone. If you are at 500/1500 MP you wont benefit it. I wouldn't go as far as to say MP is totally useless, because it does has its uses.
Lets say Mage1 has the absolute best nuking gear in the game. Then here comes Mage2 who has a lot of MP gear on. Mage1 only has 1000 MP while Mage2 has 1500 MP. Who will end up doing more dmg? If Mage2 switches into Mage1's gear
after they use up their extra MP then Mage2 would end up doing more overall DMG.
Mage1 (1000 MP)
Start<--------------------------->Stop
Mage2 (1500 MP)
Start<--------->Switch|Start<--------------------------->Stop
But if you have an insane amount of refresh and will never run out of MP for long... its best to forget about the MP thing most likely as you'll end up falling behind on DMG. In other words, if you don't really run out of MP then MP gear is completely useless~
Basically you gotta ask yourself how often do you rest and if you rest to full. MP should be one of the last things you work on as any mage be it WHM or BLM or RDM or BLU because your core qualities are what matter the most.
This isn't just about Abyssea either.