It doesn't come without costs, and I'm not just talking about the MP cost.
At the same time, let's not totally discount the MP cost either. It costs 48 MP, which is similar to adding one more person to your Haste cycle. The MP adds up very quickly. Over a 30-minute fight that's 480 MP. Auspice is not cheap; at least, not barspell cheap.
I don't want to be running in casting a spell (that does take noticeably longer to put up than a Barspell does, and also will not really be saving any lives like a Barspell would) during a moment when I suddenly might have to shift gears and throw a Cure 5.
I do concur with this statement, and it's why I think Auspice isn't necessary on fights with particularly nasty AOE. I used to do it anyway, waiting for a gap in the damage so I can run in and cast it before running away again. It's viable, you can do it, even on really nasty stuff like Cerberus if you're smart about it... but it's risky no matter how good you are. A risk that I ultimately decided I wasn't comfortable with. Not because of the risk of being hit by AOE while doing it, but because of the risk of the party needing healing while I'm doing it.
I recall one particular Jailer of Love where I was in the DD party, and just as I had run in and hit the macro to start casting Auspice, a DRK in my party pulled hate and started getting raped. By the time my Auspice animation was done to the point of allowing me to start casting another spell, this person was already at less than 150 HP. I tried to Cure him but never even got the cast started. He died too fast. Even Martyr wouldn't have been fast enough. I then took an AOE to the face from still being in range because I never had time to run back yet. Sometimes it doesn't matter how good you are, if things happen at a bad time then there's nothing you can do. The more proactive spells you have on your plate (Haste, Barspell, Auspice, etc), the less responsive you will be with reactive spells (Cures, -na spells, etc). There is a direct impact on your ability to save lives.
In short, if your melee isn't loaded with Subtle Blow already, Auspice won't really change much.
If your melee is already loaded with Subtle Blow, adding Auspice... won't really change much.
It's just an ineffectual spell.
Of course it isn't an extremely effective spell, but it does work. It might not delay his first few weaponskills, but as the battle goes on you will have significantly less weaponskills to deal with if you use this spell frequently. Think of it this way: If everyone has Auspice up, one out of every 10 weaponskills the enemy does will never happen. I'd say that's worth the effort in most cases.