I'll tell you why I use 15X binos. Last hunting season, I started using 15x56 Swarovskis on a tripod for long distance glassing. Using a good tripod head like the Slik SH-707E, I can glass continually for 1-2 hours at a time without any eye, neck or shoulder strain. Using a spotting scope, I could last for about 20-30 minutes max before needing to take a long break. Using binocular vision, I'm convinced I see as much detail at 15X as I do at 20X using a spotting scope. The FOV is bigger at 15X, so I can scan faster. With binos, I can scan in two directions using the bridge of my nose to nudge the field of view, whereas for a spotting scope I can only nudge in one direction (right to left). Using binos on the Slik head, glassing is hands-off most of the time. For glassing large areas, I'm convinced that the big binos are the way to go.