I like the idea and have done it on one vehicle but keep in mind that on newer vehicles with tire pressure monitoring you will need get a tire pressure monitor if you are planning to do a 5 tire rotation. I also got a programmer for the TPMS to facilitate changing them out.
I have used the 'best out of the old set of 4 is the new spare' on some vehicles with success but I have seem many new vehicles that use a different diameter rim for the spare, forcing you to either fork out the money to replace an essentially unused tire every few years or risk a failure as some have described when you do use it. I have also seen a lot of unused spare tires get pulled out of storage only to find that they too are flat.
A lot of new cars are just giving you a compressor for minor leaks and a phone number for road service for major issues. As few flats as I have had in the last 20 years and given the fact that I always carry a cell phone now I am on the border of saying that is the way to go these days. I hope that it works because my motorhome doesn't even have a place to carry a spare tire.