Andy,
Imagine this: for purposes of illustrating this, put a 1" spacer between the servo arm and the control inkage (ball joint). Now hold the control surface so that the linkage can't move and try to turn the servo arm. What happens to the arm? It will try to twist. If the linkage is perfectly in line with the arm (like it is when you use a clevis), this doesn't happen. It simply stalls the servo. Does that make sense? The twisting will eventually cause a plastic arm to break. Trust me! I've seen it happen in as few as 100 flights. A wheel is a better bet or even an aluminum arm will work. The thin plasic ones will eventually let you down.

John Pavlick
Team Black Magic, Tech-Aero Designs