My own past experience with a BLM/RDM (JP Player) in DA, exp'ing off of Bark Tarantulas (This was about 6 months ago?)
DRG/WAR NIN/WAR DRK/THF RDM/WHM BLM/RDM WHM/BLM 3x JP, 2x NA, 1x EU
We (our party) never needed the BLM to sub WHM. In fact, it was better that our RDM was able to sub WHM instead. There really was no need for escape from the RDM, because we camped right next to the exit leading outside of DA into Zi'Tah. Also, in light of that experience, I can say without a doubt that the WHM could've subbed SMN and maybe RDM (For fast cast, as I will explain below)
I was the party puller, although I made sure not to use ranged attack with a Bomb Core in my slot.

The DRG had an Optical Needle in her ranged attack slot and I didn't want her to pull with that either.
As I am pulling the Bark Tarantulas, even while carefully tabulating the pull rate and the subsequent repop rate, I sometimes accompanied by the BLM/RDM on these pulls. Now, you might think, this is unheard of. Why is he not resting? Well, MP was never an issue and again, I'll explain why ...
After the spider gets to camp, NIN will take aggro. RDM will then enfeeble while BLM uses elemental enfeebling. We had two sets of Divine Seal + Erase for those nasty slow-ga, but we had hasting enough to go around. At this time, the BLM/RDM, if he was accompanying me, would take a kneel, while the RDM will do some light damage in his place. WHM will usually be in charge of healing which is normally light due to having a blink tank.
Our SC setup was SATA light on the NIN. On MB, the BLM/RDM, due to Fast Cast, usually lets us start the SC and then casts AM (Yes, Ancient Magic spells) the casting time is generally fast as a 3rd tier -ga spell. If the BLM was stressing for MP, he would cast a 2nd or 3rd tier -ga spell and finish off nuking away.
You may think that AM is wasteful, but it actually conserves MP in the long run. The damage from the MB is quite powerful and a well equipped BLM with additonal stats and magic power from a RDM sub can put that through the roof. That punch from an extra expended 80 MP usually ends the fight then and there and allows the BLM to rest, instead of having to cast additional spells should the monster remain alive. Also, it allows us to SC immediately, right into the fight at times and quickly seize Chain 5s and a couple of Chain 6s. Usually, the BLM will have so much MP, that at times it is unnecessary for him to constantly take a knee. This allows him to accompany me on those long pulls and to cast sleep in case we pull a link or add from a newly popped spider. Also for those long pulls with flies (We use this to lead off the exp chain, since the spiders are more closer and easier to melee on)
I think that had the party been allowed to continue (Throughout the time there, we were on a time limit of about 2 hours, since many JP players seem to impose time limits in order to generate exp efficiency) we should've all leveled once and lead halfway into the next. It was fun as well as it opened my eyes to the possibilities of BLM/RDM. There was no noticeable problems with it and the side effects were easily negated with an accomodating support mage.
That's my experience, anyway.