I could probably get by with one for transportation/commuting purposes, but it would be a hassle up here in the land of ice and snow (Wisconsin). I find it easier to have my fast but crappy single speed bike for good weather and an old MTB for winter; this means that I do not have to constantly change out the studded tires in the fall and spring. The MTB is also good for trailer hauls, which have become frequent since I dumped my car five years ago.
In real life, I got bit by the road bug and also have a crabon road bike and aluminum gravel bike.