Personally, I think it's a great idea to have a sturdy set of winter/commuter wheels that you can just slap on your bike when the season changes. One set of wheels for faster fun summer riding, one for nasty winter commuting.
It's convenient, saves a ton of money, storage space, and extra bike upkeep. If you get the whole set of wheels (as opposed to just getting winter tires) you don't even have to swap out the tires and tubes.

On the other hand, if you want another bike for a whole different riding style or purpose, then that's different. For example, if you want a light go-fast bike for group riding or competing, and then also a touring bike that will carry heavy loads and be comfortable on long distances. Touring bikes, commuter bikes, and hybrid bikes with racks and bags can all be used as errand/grocery bikes as well.