So they completely forgot about spirits when it came to updating their spell list post 75...
(I'm assuming they still haven't changed that? Although wiki says they randomly gave light spirit an extra tier of banish and holy post 90, it lists no changes for the rest of them. Why they would give only the light based pet new nukes is an entirely unrelated mind boggle..but to be fair I haven't cast any of them, except for siphon, in years.)
...but they remember spirits when it comes to adjusting a merit that 99% of SMNs don't have anyways and still won't bother with. But, even if every single SMN had spirit cost maxed out to 5/5, this adjustment wouldn't affect anyone, because no one in their right mind would keep spirits out regardless.
But this goes a step beyond just 2 degrees of uselessness. Even assuming that every SMN ever had this merit maxed out, and that we all kept our spirits out all the time, this adjustment would still be useless because with all the new gear they have been throwing at us since abyssea, it's almost certainly possible to get a positive tick with spirits out already, with or without merits, and while an even greater positive tick would be nice, it's not extremely important because you don't have to spend mp on blood pacts when you have a spirit out.
/rantoff
Don't get me wrong, I'm happy that it's at least not detrimental, and most of the job adjustments recently have looked kinda "meh," so I didn't expect anything good. I just don't see why SE insists on being so utterly, unbelievably, and remarkably stupid when it comes to SMN.