Yes yes. And it makes perfect sense in most cases (commuting/week-end rides etc.)

But I am motivated by the special/unusual case of a 4 000 kms tour across Europe. As there are 4 of us, carrying spare tires isn't trivial. Kids won't carry luggage so we expect their tires to last the whole trip. Will be different for my wife and I. As I wrote earlier in this thread, the expected life of Schwalbe's Marathon supposedly lies somewhere in the 3-9 000 kms range. That could mean that we'll burn through our rear tires while having thousands of kms left on our front tires. I will very probably rotate tires on my wife's and my bike, expecting that we'll be able to ride the whole course on a single set of tires. I'll carry a spare and we'll buy under way if we have to.