To be honest, 10 attack isn't enough, and with capped skill/merits and other Summoning Magic skill gear makes your accuracy pretty high enough as it is. Magic Attack Bonus actually multiplies the damage, so that is a good choice.

For me, though, I merited 3 in attack (Avatar melee ftw), 2 in MaB, and the rest in Spirit cost. My reason? When I was doing my Evoker's Ring quest a while back, Leviathan managed to get a lucky 1.4k Tidal Wave off on poor Carbuncle. Let's just say Earth Spirit seemed pretty attractive with a one second cast time, but the cost to maintain! Ew. Quake, Assault, Stone IV. Dead Levi.

Also, I don't like losing MP on the off chance I don't Siphon before the tick. What is the cost of Spirits now, 19 a tick...? Am I the only one who Siphons at the last minute? lol

But, I ramble... I wouldn't call Spirit merits a waste, Mala! My damage looks exactly the same as other Summoners who are 5/5 Attack and MaB, so Spirit perp. is the only rational thing to do.