RDM Sub?

[Alukat]

one word, reraise

[Areola]

one word, reraise

i don't that's the kind of answer they where looking for <_<

some of the pros would be
MAB for WS's such as Primal Rend and lol Cloudsplitter (in addition you will make full use of the fencer job trait)
En-spells for added damage. defensive spells such as bar-spells, protect, phahanx ect.
Convert and Refresh for better mp management

Cons might be
Lack of duel-weld
no status curing spells like praralyna silena ect.
cuer3 is a little underwhelming

I would put more but i don't use /whm or /rdm personally. I'll leave the more in depth advice for someone that that has more experience in this area. ^^

[Tsukino_Kaji]

one word, reraise

One word and one contraction. Don't die.

[Xilk]

I typically go /sch if I'm going to use a mage sub. I only rarely pulled out the /whm for teleport spells doing a quest recently.

/whm you will get cure4 and reraise, plus status removal spells

/rdm you have enspells and phalanx.. and your magic skill will suck.
I would only really use it for magical WS... ie brewing on bst.
It has many other useful spells as well, but mostly you'll have better or more powerful options w/ /sch or /whm