To your questions, Anima:
So far, if a unit was a dedicated SP generator, that is, if you dropped it, and outside of requiring an obnoxiously large number of spheres (10 total for marduk/urgrant), it had an action skill of generating SP, that was all it did. You dropped it, and you used its action skill. You didn't tank with it, you didn't attack with it, and you most certainly didn't lock out rats and succubus with it! Basically, if it genned SP, it was already a threat in and of itself because it'd eventually turn the tide of the game by genning SP, eventually paying for itself, and when it died, give its level in SP back as well.
Marduk was an exception to the rule because she needed 10 spheres total. 4 to summon her and another 6 in refess to use her SP generation ability.
Does mid ref have bad matchups? Yeah, and those bad matchups are ones in which its DF buffs are either punished (shrine knights, carbuncles), or ignored (orks, Dio control), and thus making their units not worth the SP because you don't benefit from all their stats. Do mid ref perform better? Well, Alleria with a Gaiessoul splash got a gold star if that's what you're asking. It's a pretty brainless file, too.
As for rushing out miff being the best solution, generally yeah, because if you rush her out, your opponent doesn't have the firepower to bring her down effectively early on. If you try to set up protection first, you're unnecessarily giving up SP because miff is self-sufficient early on thanks to her massive 55 HP.
As for multisphere files, well, I once ran a falkow/refess marduk miffyre file. I put it down because playing it was torture. Its damage output was low, and the game revolved around setting up TR and just slowly grinding down my opponent (and my own sanity).