Except that this is a massive waste compared to anything else you could spend the merits on (that's what they should cost in the first place...). All the other merits increase effectiveness. This one just reduces MP cost. Aside form the fact that the spirits casting spells are massively inferior to anyone or anything else casting the same spell.
Uhm, you couldn't be more wrong.

Pet attack does nothing. 5 merits in it will roughly give you 5 more damage on your BP:Rage. It is so useless that you'd only pick it if there was nothing else to pick.

Pet accuracy does nothing for BP:Rages if you got enough skill over cap. At that point it only helps pet melee, and I don't know how much 15 accuracy helps that, but I have never seen any melee jobs stop at +15 accuracy and say that now they'll eat meat.

Pet magic accuracy does nothing as well. I've had merits in it, and my spells were as resisted as always. Summoning over skill helps more, and again is the way to go.

That leaves pet MAB, as a good investment for 5 merits. And your last 5 merits can either be wasted on the above 3 useless choices, or invested in spirit perpetuation. I went with spirit perpetuation since it actually lets you use them more freely than before. (Even if you can hardly use them at all)

Back when 75 was the level cap I actually used spirits for quite a great deal. With 5/5 elemental perp cost merits I found spirits at that time to actually be better then carby or other avatars for the long pet kiting solos.
I don't know what you were fighting, but I dozens of bad memories from sicing my spirit on a EM mob, just to have it interrupt every single spell that was attempted to be cast. With skill past the cap I may add. It seems to me the only monsters you can use spirits on, are casters, since they stop meleeing your pet to cast spells and allow your pet to cast a spell back.

It is a bit unfair that you can hit an elemental 20 times and not interrupt them, but a EM mob can just hit your spirit once and ruin your spell.

Alhanelem. I'd like to point out a typical scenario that spirits also can't save you from.

You are in a group of aggro monsters, and one accidentally aggro you while recasting sneak. You toss up spirit to save you, and you are saved. Now you cast sneak to keep walking.... BAM. Monster is on you due to hate from sneak spell. At this case your only savior is if your spirit decided to cast tier IV, else you are going to die.

I will add that I've never survived getting aggro in a group of aggro monsters just because of this. I only survive if it is a single aggro monster that I can run safely from and then start killing it or zone.