Third-party diagnostic software - a sound investment?

With Windows XP (especially) is it necessary to purchase third-pary diagnostic software for service and repair work?

Searches I've done reveal that most have a pretty hefty price tag. One expert is on record as mentioning (jokingly perhaps) that they look very impressive to clients when they are running and actually buy him some time to figure out what is really wrong with a system.

The question is, do regular techs really have these in their tool boxes and should useful diagnostic tools be testing both the hardware and software?

I'm thinking that if a system is really broken then software of any type probably isn't running on it and is this a factor in decided how much to invest if anything?

I am obliged.