What is the advantage to purchasing a true rms clamp on meter?

TRMS meters give more accurate readings in the presence of harmonic currents or non-sinusoidal waveforms, such as the output of VFDs or SCR drives.

From what I've seen, you could be troubleshooting a breaker tripping, intermittent problem ,and without a true rms meter you may get inaccurate readings throwing off your diagnosis. Provided it's a 3 phase system and there are harmonics to contend with. If you work in commercial or industrial buildings, definitely get a true rms meter. Averaging type meters are only good for residential work in my opinion.

It's not an "either/or" issue.

For 90% of your meter work, a cheapie will work fine. Then when you are trying to identify harmonics, you will use both to identify the problem.....if the "True RMS" reads more than 10/12% higher than the "cheapie," it is time to start considering harmonics as a problem.

Yes, that's what I was referring to. My point was that 10-12% difference that you may see between readings is enoughto throw a guy off who's not aware of the influence of harmonics. I worked with a very experienced electrician who only believed in cheap meters. There was a few times that he misdiagnosed problems due to his meter reading a lower amperage than was actually there due to harmonics. His meter would read a lower amperage than my true rms fluke. He would never believe me cause I was the rookie and he the journeyman. Most times we were not given time to diagnose the cause of harmonics on the line, these were commercial accounts with "not to exceed" limits. "Just fix the problem with the sign, etc, tripping the breaker, that's all, pull a new circuit if you have to, just get in and get out". I agree with you that a good troubleshooter should have two meters, an averaging meter and a true rms.Good point.