I would like to buy a gently used OEM wheel to match my others, so that when I replace my tires, I can rotate in a fifth tire/wheel set. This is to avoid having to replace all tires if one is destroyed prematurely, as was the case with the OEM tires.
I can spread the miles to five tires instead of four, and if one is damaged I can continue to run out the tread on the remaining four.
Does this sound crazy, or have I figured out something useful?

I guess that concern might be asymmetric tire wear. The front pair will wear at a high rate, the rear pair will wear at a reduced rate, and the 5th wheel will be significantly off from either of those pairs by the time your first rotation comes along (if rotating at typical 4-5K mile rate). Not to say having a full size spare wouldn't be helpful, just not going to be easy to manage it such that all tires are fully consumed by the end of their useful life, and even harder if your tire's tread requires a wheel remount to move from left to right side or vice-versa. Now, if you had SIX wheels, you'd be golden.

Thank You,. I see your point about the odd-man-out tire wear.
If I were to rotate tire #5 into a front position at (lets say 1500 miles) while simultaneously rotating both fronts to the rear, and a rear tire out, and doing this consistently every 5000 miles thereafter,......
This isn't the solution. But I'm sure there is a good mathematical solution to this problem.
Let's say the front tires wear twice as fast as the rears. I think this is probably about correct for my driving habits. There has got to be a mathematical formula for rotating in the fifth member and minimizing the wear difference between the remaining front and rear pairs.
So, get to it young people. I have been out of college for too long to figure this out in a reasonable time period. Maybe some of you are puzzle fanatics and know how to do this off the top of your heads.

I have a set of 4 in great condition if you come to the desire of buying another set...they came from a 2007 MS3...they have less than 20,000 miles on them and have TMPS in them right now. The tires are about done though.