It's a roulette really. I've had it complete bone me over where I've been stuck with a group of 3 melee characters with no way to hit ranged targets.

But on the flip-side, the SPP members do count toward "Kill X enemies with Y characters" bonus objectives at the end of missions. As you achieve more and more of these you unlock weapons and costumes for those characters that can be bought with SPP.

Admittedly most of the stuff can also be bought for Gil too, though some of it is quite expensive gil wise.