You don't mean packing the spare around the waist?
I think it really would depend on the ride. When I was riding on sewups, I would pack several of them for a long ride.
If I was heading out on a month or several month expedition, I might consider packing a spare, especially if I thought my favorite tire might be tough to acquire on the road, or I had enough miles planed to grind through a tire anyway.
Otherwise, for a few hundred miles, I would probably only bring a spare if I thought one of my tires on the bike already had critical damage, at which point one should probably just replace it before heading out on the road.
I've now caught the weight-weenie bug, and am trying to think how to minimize touring weight/bulk, so that alone might nix a spare.