I've been commuting year round for close to a decade. I have two bikes but only one is suitable for winter riding. So during the winter, which for us is 3 to 4 months, I have no backup. For me it is mostly about space, but part of it is time commitment, and part of it is money.

Studded tires are not cheap. Further. I'd rather not spend the time maintaining or pay somebody else to maintain a bike whose job it is to sit there just in case. I can ride public transportation for free, but if I did have to pay, it's about $4.00 a day. That's less than it would cost me to drive and park a car. Finally, like a lot of us, I usually do have a car available if all else fails. For that matter, I've even run the 6 miles to work before.

And like I said, a backup bike does no good if there's a mechanical issue on your way to work. It's more important in my mind to have a plan for that situation than to have a backup bike. Yes, if you crash and your bike is out of commission for awhile, a backup bike would be handy. But what if your ankle is broken in that same crash?