While there may not be a difference in calories expended, there is a difference in muscle-exertion effort. This may not be that big of a deal on a leisurely 4-hour ride, but can make quite a difference in a TT or competitive situation. If you're trying to ride as fast as you can for 1-2 hours at LT, then using 100rpms vs. 80rpms will make a big difference in fatigue and cramping.