Stu, fix flat is a handy quick fix that can get you to a garage, but it doesn't always do the job. If you are out for a drive with your sweet bride enjoying the day and dressed in nice clothing, you may appreciate not having to get the spare, jack and chocks out (chocks -I would encourage you to carry these) and getting all grimey wrestling the flat off and the spare on. Think of it as a first line of defense.
I would encourage you to go through the exercise of changing a tire before you have to. I did this in the relative comfort of my garage and found that the bottle jack Steve supplied was inadaquate. I chocked the wheels, jacked the car up and took the tire off. After a few minutes, the suspension relaxed, elongated and I could not get the tire back on. I had to use another jack to get the car back up high enough to get the tire back on. I now carry this one in the car. It fits neatly in the well in front. I believe that it's a 2 ton hydraulic that comes in a nice plastic carrying case. Chalk one up for Wal-Mart.
Hoss