As long as the vocals fit with the music they never bother me, singing wouldn't work on a grind record, and growling would sound shit on a power one. Not too difficult is it.
Grindcore is mostly really extreme hardcore with constant blasting, it often has left wing political veiws as well, good examples are: (early) Carcass, (early) Napalm death, Brutal truth, Repulsion, Nasum, Pig Destroyer and Exit 13.