Regarding the /nin on the mage job
http://www.datasync.com/~dsmith/FFXIStats/
Figuring level 30 blm with subjobs fully leveled here are your base stats for blm/nin blm/whm blm/sum I'm using elvan as the race since thats what I play with and I've played with the whm and sum sub for experimenting.
Level HP MP STR DEX VIT AGI INT MND CHA
blm/nin
30 437 225 30 31 28 29 31 30 28
blm/whm
30 430 259 29 28 27 27 31 34 30
blm/smn
30 414 285 28 29 26 27 33 33 31
Ninja gives you the ability to have an extra wand with +5 int and mind at the loss of healing ability. You also get shadows but if your riding the hate line properly they shouldn't be needed.
blm/whm is the most common combo to xp it gives you the ability to throw status repair and cures but has the same base int as nin at this level.
blm/sum gives the highest base intelegence, but is only by 2 at this level and the highest mana going from 225 as nin to 285 as summoner. Unfortunatly the lack of curative abilities and summoning magic at half strenght only being fit for carby pulls and at higher levels buffs like arial armor reduce the desirability of this sub as well.
Usually your best bet is whm since it gives the most bang for your buck in utility.
Sidenote: For the love of all things holy I realize it's fun to melee too, but that is not a mage jobs primary duty. Please don't bring /war or /nin to xp as a redmage /blm or /whm or even /smn add so much more to the job when in a party.