My guess is that they are checking them for a trend. Those capacitors are cheap if a service guy is standing there with one on the truck. If that capacitor fails, the whole unit will fail, and you will be paying for another service call. Any time I have replaced them, they have already failed.
So, when it comes to stuff like this, you have to know the cost to replace then, and the cost if he has to come back. In your case, it sounds like you sent the guy on, so the advantage of replacing a part because of it showing failure signs is almost gone. If your luck is like mine, it will fail so you wind up paying double overtime after a 12 hour wait on a 90 degree day. So, are you sure you know what a capacitor does? :) I think I paid maybe 8 bucks for the last one that went into my unit, but I replaced it myself. The HVAC guys will mark it up some