I agree grindcore songs are too fucking short, although grindcore is my favourite genre after death, black and doom metal. I reccomend Lust Of Decay, Extreme Violence, Haemorrhage, Saprogenic but still my favourite grindcore band is Fuck...I'm Dead.
I think longer songs would make grindcore be really annoying. I think the short songs make it better. I enjoy Fuck... I'm Dead, Pig Destroyer, and He Who Corrupts.