Mainly so that the bike can have a shorter top tube without compromising design in other areas (like a steeper seat tube angle or greater fork rake). Most of Terry's bikes below a size 19" have two different sized wheels. My 19" Terry road bike has two 650c wheels for the same reasons.
When I'm on the bike, I don't notice it at all. I just think it makes the bike a little 'odd' looking. So far, my only two issues with it are 1) I want to buy lighter wheels and they are exceedingly hard to find and 2) it's difficult to find fenders that work (I'm currently using race blades). It is definitely a huge conversation starter...kind of like my Mini was when I first got it.![]()
My new non-farm blog: Finding Freedom