My preference is 10x binos plus a spotter. I have a Swaro 65 I bought almost new on this websites classified section. If you hunt open country and like to field judge game it sure is nice having a spotter. I can also spot a rump, anter tip, etc at long range that would be impossible to see with 15x binos.
15x are nice for glassing and spotting game similar terrain/vegetation such as brushy coues deer country. I had a sheep tag a couple years ago and it would have been nice to locate sheep on a huge rocky ridge that was around 1 mile long across from me where a spotter was too tight and my binos tougher to spot a ram horn or rump in cliffs.
I do an incredible amount of long range spotting and field judging with my spotting scope and there is no way I would leave home without it. It has saved me miles and miles of hiking to get a closer look at game.
Something else I would highly recommend is a bino/rangefinder combo. It's a lot quicker and easier using just a combo. Holy smokes my Leica combo is nice! I got a nearly new demo pair for a chunk off new price from cameraland.
To answer your question, if you can afford it I can see where 10x plus 15x binos plus a spotter would be nice. As Fatcamp mentioned, I definitely wouldn't want to carry all of them. If I had all 3 available I would pick 10x binos plus my spotter 95% of the time!