Probably a little history would be good here.
A/C started to become really popular in the early 60's about the time that muscle cars began to emerge. Most factory hot rods didn't come with A/C at the biginning. As muscle cars developed, and A/C also became a bit more refined, muscle cars with A/C became common, but often without an A/C option for the highest performance level cars. A prime example was the L88 Corvette. No A/C, not even a fan shroud so that they would overheat if driven in heavy traffic. After all, such cars were intended for track use and were not expected to see street use.
Hot rodders, being the inovative people that we are, took these muscle cars (as well as others) as developed for street use and modified them with hotter cams and lower gears. It is these cars, lower geared (higher revs at cruising speeds) and before A/C systems were designed to run in the defrost position that most of the problems came from throwing on the A/C at freeway speeds the first time in several months. I saw a lot of them.
It sometimes still happens today as well, but not nearly as often.
Remember that our cars with manual transmissions are geared a bit low so that at freeway speeds you are often at about 3500 RPM. That is a lot more risky than the cars that are geared so high that they are only doing about 2200 to 2500 RPM at freeway speed.
So it is my personal precaution to avoid turning on the A/C at freeway speed. Even if you don't have the compressor throw a rod, you will have less wear and tear on the system.
