If you aren't concerned about field judging animals a spotter likely isn't necessary. If you are a trophy hunter where field judging is important...read on!
I often hunt relatively open country. One common way to hunt is to sit on one side of a canyon and glass directly across to the other side of a canyon. I'll first glass an entire canyon face with binos to cover it fairly quickly. If I spot an animal I'll size it up with my spotter. If I don't spot any game I'll still grab my spotting scope on tripod and glass the canyon where game tends to feed, bed, etc. I'll spend time glassing in detail the better areas that generally hold game with both binos and spotter. More times than not I'll spot a tine, antler, rump patch, ear with my spotting scope that I miss with binos. I'm not sure how many times I've run binos over country....then switched to spotting scope and found game! A spotter has saved me miles and miles of hiking trying to get closer to field judge animals with smaller power optics.
There are several problems I see with 15x. They are bigger, bulkier, and heavier than 10x binos. I definitely don't want to carry both 15x binos plus a spotting scope plus tripod. 15x are impossible to hand-hold without a tripod. There is no way to still hunt and glass with 15x. 15x are too small of power to field judge animals compared to a spotting scope. You would need 2 tripods....one for 15x binos and a 2nd tripod for a spotting scope unless you are willing to switch back and forth with 1 tripod. You will have to stalk a lot closer to game to field judge them if all you have is 15x binos and no spotter. You may miss animals with 15x that a spotting scope with higher power will pick up.
With that said, my preference is an 8 or 10x binos for spotting game and a spotter for sizing them up and locating game at long distance. The advantages to 15x have been mentioned in other posts above. I guess it's up to you which route works for your particular style of hunting and the country and animals you hunt. Again, if size doesn't matter a spotter may not be needed.
I often hunt relatively open country. One common way to hunt is to sit on one side of a canyon and glass directly across to the other side of a canyon. I'll first glass an entire canyon face with binos to cover it fairly quickly. If I spot an animal I'll size it up with my spotter. If I don't spot any game I'll still grab my spotting scope on tripod and glass the canyon where game tends to feed, bed, etc. I'll spend time glassing in detail the better areas that generally hold game with both binos and spotter. More times than not I'll spot a tine, antler, rump patch, ear with my spotting scope that I miss with binos. I'm not sure how many times I've run binos over country....then switched to spotting scope and found game! A spotter has saved me miles and miles of hiking trying to get closer to field judge animals with smaller power optics.
There are several problems I see with 15x. They are bigger, bulkier, and heavier than 10x binos. I definitely don't want to carry both 15x binos plus a spotting scope plus tripod. 15x are impossible to hand-hold without a tripod. There is no way to still hunt and glass with 15x. 15x are too small of power to field judge animals compared to a spotting scope. You would need 2 tripods....one for 15x binos and a 2nd tripod for a spotting scope unless you are willing to switch back and forth with 1 tripod. You will have to stalk a lot closer to game to field judge them if all you have is 15x binos and no spotter. You may miss animals with 15x that a spotting scope with higher power will pick up.
With that said, my preference is an 8 or 10x binos for spotting game and a spotter for sizing them up and locating game at long distance. The advantages to 15x have been mentioned in other posts above. I guess it's up to you which route works for your particular style of hunting and the country and animals you hunt. Again, if size doesn't matter a spotter may not be needed.