5 tire rotations also mean that if you need to use the spare for it's intended purpose, it's diameter is always roughly the same as the tire it replaces avoiding potential stress on the drivetrain.
How many miles would be too many to throw the spare into the mix? I had weirdly worn ko2's recently and just putting on a lift gave me some death wobble since the tires didn't handle it well. Of course, the install place told me this so not sure if its actually true. But It seems to me if you have a decent amount of wear on 4 tires, throwing in a new 5th might throw things off a bit and cause some vibrations? This could be said with waiting too long on a 5th tire rotation as well. I'm assuming if you do the 5th tire rotation... you have to rotate at least every 3-5k to be affective?