Less noise, real use of engine torque and flywheel effect, mind serenity. Why howl when you can whisper?
So far I'm liking the change to 15T. It's nice to be able to just let out the clutch and not have to consciously feather it. 1200 rpm is 7mph, that's the bottom limit for "riding". I still frequently have to go slower in traffic, so that's a concert of clutch, rear brake, and tap dancing, but a little less of it than before.
Less noise - With the OEM, rubber-dampened sprocket, theoretically, there should be less drivetrain noise. I wear ear plugs, can't tell.
Real use of engine torque - According to this chart, the DL650 makes peak torque between 6000-7000 rpm. At 75 mph actual, I'm right there in the heart of it, not below as you would be with a higher final drive ratio. Of course, if you like to go faster, gear up. The bike definitely feels quicker with lower gears. That is expected. Every single ride, and usually many times during a ride, I have to accelerate from a stop. It is much less often that I find myself at 75mph or faster. That's where my need is, so that's where I bias the bike's performance towards.
Flywheel effect - I have no idea how big the 650's flywheel is, can't be much. I'm probably jaded by driving a Dodge/Cummins diesel with an NV5600 transmission that makes almost 800 ft/lbs of torque on the dyno at 1700 rpm. My perception of torque and flywheel effect is no doubt skewed by that.
The other arguments are even more subjective. I'm happy and my bike is quiet.
My opinion is not an indictment of 16 or 17-tooth countershaft sprockets. Having tried 17, 16, & 15 (in that order), I just find stock gearing more suitable for my riding needs. I have yet to do my monthly 500-mile weekend on the 15T (last one was on the 16T, never did the 17T), so we'll see how much of a compromise the stock gearing is on some longer stretches. I expect the verdict is that I choose to make my compromise there rather than on the daily grind.
To each his own.