I do a five-tire rotation because I have a matching spare and want to make sure that, if and when I have to use it, the wear pattern roughly matches those of the remaining three tires.
Mixing a spare with fresh rubber with tires that have substantial wear risks the issue of compromised traction and handling, which I'd rather avoid at both highway speed and off-road. This issue is due in part to the difference in tread wear, and in part because a new tire will still have its coating of mold-release lubricant on it from the manufacturer's curing process. It takes roughly 500 miles of break-in before a new tire's coating is worn off and it performs at full design spec. In other words, a fresh spare will usually have inferior traction to the three broken-in tires. Having a spare with fresh rubber is better than having no spare at all, but a five-tire rotation avoids this issue and isn't any more difficult than a four-tire rotation.