I guess for me it was pure aesthetics. I always hated the way a mountain bike looked when you put skinny road-type tires on 26 inch wheels, like you have 24 inch wheels on a 26 inch bike. also I hear people complain about geometry and bottom bracket height being affected but I believe it’s nonsense. It’s the overall diameter of the wheel with tire mounted that matters. My 26 inch wheels with 1 inch roads slicks are 1.5 inches smaller than a 26 inch wheel with a 2.1 mountain bike tire.
The 700c wheels with the cyclocross tires are only a 1/2 an inch taller than the 26/2.1 combo, and with road tires they are the same.
Finally, as most mountain bikers who switched to a 29er will tell you, the larger diameter wheels make for a noticeably smoother ride over rougher surfaces, while the 26/skinny tire combo makes it worse.
...so let’s see, I had all the parts on hand except for the rear brake and tires which I spent a total of $80 on, The bikes geometry and handling characteristics are far less affected then they would be with a 26 inch/skinny tire combo, I didn’t lose any top end gearing, and the wheels don’t look too small for the bike.