Requirement and function over fashion for me. I run different tyres because the one on the front of my bike has 4855 miles on it, and the same tyre on the back didn't last over 700 miles, so I use different rubber on the back.
My back wheel is also different now (Velocity Aerohead onto an IRO flip/flop) because I can't get the 17T cog off my Formula/CXP22 wheel, and I changed cog to a 15T.
I needed to change wheels to change ratio, I didn't really care about the way it looked - I just wanted to ride my bike.
edit: I should note my wheelset was originally black rims/spokes, my new rear is all silver - extra mismatch power!