With cars, and above a certain latitude, it's common to own one set of wheels (with fancy/light rims) for summer and another set (with ugly/heavy rims) for winter.

So far I stop cycling from mid-November to mid-March. I am now considering continuing to cycle through snow season. The 700-23mm tires are clearly not an option. I am also now rather partial to drop bars, and so I am contemplating buying a cyclocross bike rather than using my mountain bike over winter.

This begs the question. With a cyclocross bike I no longer have any need for my mountain bike. But why would I bother keeping a road bike if I have a cyclocross bike? They are both of similar weight (just under 10 kg) and as a bonus I'm aiming for 105 components, an improvement over the Sora on my road bike.

I could change the tires twice a year, but I could take a hint from cars and do better: Buy a new set of (lighter) rims plus 23 mm tires for the summer. The cyclocross rims seem to be heavy anyway, and so there is another justification for doing this. Switching from summer to winter would then be a (relative) breeze. It's understood that I would need a second cassette.

Why maintain two bikes for summer/winter rather than two complete sets of wheels (as with cars)?

As a (very) amateur bike mechanic this would have the added advantage of reducing my maintenance burden.