Sorry Circ, I gotta side with Eric on this one.

No logic involved ... just preference.

Well, maybe a little logic:

You drive a vehicle that can apply drive to all 4 wheels. That vehicle is designed (within reason) to do so with tires that match. If the tires don't match then obviously you are causing the drive system to work harder to compensate for the mis-match. You'll either cause a clutch system to slip more or a viscous coupling to slip more. Either way (in our case it's a clutch system) it will cause additional wear ... either clutch wear or additional heat generated by the viscous coupling. The bottom line is that if your tires don't match, it may not be unsafe, it may not damage the vehicle, it WILL cause accelerated wear of certain components.

That's all I got to say about that.