I have always heard 15's are great for long range spotting in open country off a tripod but have yet to try it. I guess I'm a bit old fashion because I have always used 10's for spotting game plus a spotting scope to field judge. Obviously you should buy particular optics that are best suited for your particular method of hunting (and your budget).
If you are still hunting and trying to hand-hold binocs the 15's would be next to impossible to use. 8's or 10s are very useful for still hunting and glassing at fairly close range in thick brush; however, I can see how the 15's would be a lot easier on the eyes and provide a wide field of view for long range spotting compared to looking through a spotting scope all day. An added benefit of the 15's would be higher power than the 10's for locating game.
I've always used the 10's for spotting game and then a tripod and spotting scope for zooming in to judge game. If I had the choice between a 10x binoc plus a spotting scope or just carrying a 15 power binoc I would choose the deadly dual any day! You wouldn't believe how many miles of hiking a high quality spotting scope has saved me over the years! I can usually tell the difference between a 170 and 180+ buck at long range with spotting scope but have a feeling I'd have to hike closer to judge if I only had 15x's? I can't imagine trying to field judge dinky antlered coues deer at long range but the coues guys best buddy seems to be 15's...I have a feeling they likely have a spotting scope on a tripod as well?
If limited to 15 power you will have a lot tougher time field judging game at long distance compared to cranking up the power on a spotting scope! Obviously if you are in a hunting situation where you are carrying everything on your back there is only so much you can carry. Conversely, if you are hunting from a truck or where bulk is no option it may be worth having all the optics you can afford and use!
Another thing to consider is what species you will be hunting? If you are hunting horned game such as mtn goat, antelope, and sheep where fraction of inches in mass and horn length is the difference between an average animal or a trophy the spotting scope is going to be a lot better for judging these critters than a 15 power binoc! I often find myself cranking up the power on my spotting scope even at short range to get a better idea of tine length, mass, number of pts etc. If a 9" mtn goat billy is considered good but a 9 1/2" billy is a true trophy...I think you get the picture?
Just a few things to ponder! I'm going to have to break down and buy myself a pair of 15's one of these days because I'm sure there are some great benefits I'm currently missing out on!