You need good, solid players. I can't tell you how many people have come and go in just our shell alone and we were blinded by the job they had -- it happens to damn near everyone.
Good players will realize that they need to be flexible to help the LS adapt to many different fights. They will also put in the extra work to research fights and their job on their own as well. That's the kind of attitude you want in your shell. Having someone stuck on just one job more often than not (there are always exceptions) can handicap you. Personally, I've found that people that play multiple jobs tend to have a better grasp on the game period because they know the strains that each job can go through.
On another side note, my favorite BLM pt setup (didn't see this mentioned): BLMx4, COR, BRD -- double ballad + COR mage buffs is pure awesome. We tend to use RDM in the tank party only.
I would say at this point in time, it's not about the MP -- It's how you use it.
/smn *only* gives a generous MP boost...
But compared to the spells and abilities granted from /SCH and /BLM it's not worth it.
Especially not with the Refresh rate going these days.
Did I come close anyone? Haven't played in a while, but that would be my take.
I can get into the math if it's necessary but 20mp/min auto-refresh beats conserve MP trait from /blm until mp-burn rate exceeds 285~mp/min average from rest to rest. (edit: this number is largely independent of gear and refresh rate considerations from refresh/ballad/conserve mp roll, etc.)
I only hit that number for entire fights during zergs, pretty much. so unless you're zerging it's unlikely that your whitemage is burning mp that fast and unless you *need* a weak sleepga (repose is better than sleep) or escape on the whm, whm/smn will tend to outperform whm/blm.
I won't bother with comparisons to whm/sch since I think it should be fairly clear that /sch is a superior sub for whm. (light arts alone is superior to most other subjob choices).
Never mind, got names mixed up.
WHM/SCH > WHM/SMN or WHM/BLM for 99% of things you do.
WHM/SMN - carby pull
WHM/BLM - elemental seal, escape
WHM/SCH - everything lol