I dunno if you mentioned this, but they actually have already gone a step inthe right direction in terms of adding animals into GTA. The pigeons in IV actually looked like pigeons and when you shot them you got blood and feathers going everywhere.. Not like in VC (or was it SA) where you shot those birds flying and they just disappeared off the sky. Sure the pigeons didn't roam the map, but it's killing animals either way.
Perhaps rockstar were just dipping their toes into the animal bit slowly.. Starting off with an animal that's considered a pest so as to anger as little people as possible.