Well said tjspiel and others. N+1 is a great philosophy to justify doing what you already want to do, but far from a hard requirement. By most of those arguments, we also need extra cars, phones, spouses, etc.

Excess winter corrosion is very valid. But unless you're sporting a super high end bike and a real clunker, I think the extra winter wear on the components is no worse than the cost of maintaining a second bike. Ride what you want and replace what needs replacing when it needs replacing.

I wound up making a winter Franken-bike primarily because I could from stuff I already had. The real benefit I notice is the flexibility to choose studded or not tires. Sure, you could get by with just a spare wheelset, or ride studs even when the streets are clear, but I don't have to and I like it. Another guy I know really amps up the lighting for the dark winter AM and PM commutes, but doesn't like to have all that kit on his bike for mid-day winter weekend rides or have to mount/dismount it, so that's what he likes about a second winter bike.