I mostly use a spotter during preseason scouting for deer and elk. It lets me peek into drainages without leaving boot prints and spooking game. While I am carrying a rifle I seldom carry a spotter.
Pronghorn hunting is a bit different. I use a spotter quite a bit during the hunt.
Whether you need one or not will depend upon the type of terrain you hunt and antler point restrictions for your unit. In areas with timber and small parks a spotter can be worse than useless. In open areas it would be nice to be able to determine if game is legal before you start on a leg burning hike.