Yeah, I'm not sure how crucial that single kryptonite grenade was, but they would sure be in a sh**ty situation if Batman didn't create the only weapon that could kill Doomsday. By accident, but still.
And he was the one who drew Doomsday to the docks where the spear is.
Anyway, I don't mind him not being of much use in the fight itself, actually I was amused by how useless he was, I expected him to have more of those grenades and weaken Doomsday gradually. It would actually make sense for him to immediately go for the spear while Wondy and Supes are fighting, but the story called for Superman and Lois having a tender moment together, so yeah, Bats did seem to stand with a dick in his hand in the beginning of the fight. Again, much to my amusement.