Whispering Wind is equivalent to either Curaga 1 or Curaga 2 (I forget), for more MP, more delay (gotta summon Garuda after all), you only get it from sub at very high levels, you can only use it once a minute, and the only significant advantage is that the avatar gets the hate from doing the healing.
Frost Armor and Rolling Thunder wear off almost immediately, though they'd be quite useful if they actually lasted long enough to say so. Shining Ruby is okay but still only lasts a short time. Aerial Armor is better than Blink (three shadows instead of two - these are still unreliable blink shadows, not reliable utsusemi shadows) and it lasts a *long* time like 15 minutes or so, but the above notes for avatar abilities in general still apply.
I'm surprised nobody has mentioned Tractor yet. Or for that matter, Blind (and less importantly Poison). Nice to be able to debuff if you don't have a RDM (or don't have a RDM worth their salt), and the BLM would rather nuke - or use their elemental debuffs like Frost, that WHM/BLM couldn't land due to half-level elemental skill. And Tractor is, ya know, pretty bloody useful if someone dies in a bad place.
If madrigal from subjob BRD does the full +15 I'd be *very* surprised. I highly doubt its effectiveness is capped even for a 37 BRD main (generally BRD buffs cap their effectiveness about the time you get their replacement - this is observable at least for the ones with observable numbers - so I imagine it caps closer to 50). And when subbing BRD you're not using an instrument, and so you're only using Singing skill for buff effectiveness, rather than Singing skill + Wind/String skill. So cut the effective skill in half and you've got a Sword Madrigal about as effective as a level 18 BRD who *is* using an instrument, and that's when you're 75 WHM/37 BRD yourself.
Ballad is, of course, full and never-varying effectiveness from the moment you get it, but it does still tie up 8-10 seconds once every 2 minutes (which either means a delay before you start resting, or it means you're tied up mid-battle and that spider had better not choose to use Sickle Slash), and you're sacrificing base stats, job abilities, job traits, and spell selection from a more otherwise-useful sub just to be able to ballad yourself. (This applies both to WHM/BRD and RDM/BRD arguments.) 1 MP/tick is nice and all but it's not *that* important, in my opinion.
RDM sub gets you Fast Cast I at 30/15 and Fast Cast II at 70/35, and it gets you Dispel and Gravity (and a few things that might be useful when soloing such as Phalanx and En-spells), and otherwise doesn't get you anything that you don't naturally get as WHM or would get from BLM sub instead. The Fast Cast effects are a tad unimpressive. Dispel and Gravity are good arguments however, though just like you wouldn't benefit from subbing BRD if you have a BRD along, you wouldn't benefit from subbing RDM if you have a RDM along.
So I can see RDM sub and BRD sub having limited situational use, and SMN sub and BLM sub being more generally useful - BLM generally offering more *utility* (spell selection - Warp, Escape, Tractor, Blind, Sleep, Sleepga and the JA Elemental Seal; throw in Aspir as well although it's half-level skill and will consequently very frequently be resisted), while SMN offers higher stats but less utility of what you can do. They each contribute to MP efficiency in their own way that varies by situation. As already mentioned, Conserve MP is better if you're casting a lot, and the stuff you're casting is fairly high on MP. While Auto-Refresh is better if you're casting low MP spells, and infrequently.
