Personally, I use a standard switch harness no mater how many servos are in the plane. I do use a Heavy Duty Switch Harness if the plane has a gas engine. I have 4 gas engines and all have a H/D harness due to the vibration that these large gas engines produce. I have never had a std. harness fail, and some are 5 years old, although I have had one H/D harness go dead in flight.
It was a kill switch. Not positive why it failed but I would bet it was from vibration. Several of the guys I fly with don't use engine kill switches on their gas engines because of the problem of vibration. In short, if it makes you feel more secure, spend a few extra dollars and buy H/D, as for me, I have a 76" w/s stick with 7 servos and over 400 flights on it and it has a std. switch harness. To each his own.