most things in life aren't "really necessary", but unless you're under severe space and/or budget constraints, having a 2nd bike that can serve as a back-up/foul-weather/winter bike for daily commuting can be a really, REALLY nice thing to have, especially for those of us who live in ice country where riding with studs becomes a pretty important consideration. sure, one can switch out wheel sets too, but having two bikes ready to go at a moments notice on any given morning allows one to more easily toggle between studs and no studs for that time of year when ice is very hit or miss.

like you say, it's not necessary, but a second bike is a luxury that i would strongly recommend to any daily commuter thinking about going 12 months in a 4 season climate.