One flew over the cuckoo's nest
The 320/120 "rule" was based on the resistance of wyrms (tia, jorm, etc). While it applies somewhat nicely to other things (sky gods come to mind), it is not really relevant for most mobs for blm. Of course there is a huge difference between nukes and enfeebling. A half resist does not matter one bit for enfeebling. The mp cost is also much lower. Having to recast will cost you another 15 mp for slow, and then 45 more for another slow2 whether slow1 lands or not. While 60 mp is a fair chunk, it is nothing compared to a 171 thunder4.
Also, a rdm will almost always have spare mp, while there is no such thing for a blm. There is rarely any reason for a blm to stand around with mp, and not spam nukes until he's out of mp, then rest. A rdm of course does not really benefit from spamming mp away (at least not on anything that enfeebling resists would be an issue on, since by then nukes would never/rarely land), so they use mp as needed, not as they have it (and almost always have more than they need).
...I'm going blank...can slow/para be half resisted? If they can, then a 50% land rate results in 75% of casts landing (50% unresisted, and 50% of the remaining half being 1/2 resists, for 75% unresisted or 1/2 resisted). A 50% land rate for blm is completely unacceptable, but for a rdm enfeebling, it is more than Aquitaine. Though even if it cannot be half resisted, a 50% land rate is still almost always more than acceptable, since you only stand to lose a bit of mp, which you will rarely need.
Creator and Leader of the Anti-Rog faction