rdm or smn

Was bored last night playing around with carbuncle did Poison Nails < Fast Blade = Distortion + Blizzard II MB then again they were too weak mobs so I dunno how it would work on EP+ :\ but was fun thing to do once I mastered it ^^

im kinda lvling my rdm right now and ive been going as rdm/smn.. but i see ppl saying that its not as useful as rdm/blm or rdm/whm.. i just want to know the pros and cons of this choice.. the only ones i can really think of are.. Pros: more mp, eventually get smn mp regen, eventually get...