Cliff, this is a very good writeup. I appreciate that you did not hide any shortcomings of XTZ software.
[Rant]It does read like you are making excuses for every issue you call out. If a free software like REW surpasses XTZ on many of its usability issues, the developers have just become lazy. That the interface looks like an antiqued Java front end for a client-server stack, is proof positive.[/Rant]
The only egregious flaw with XTZ is the lack of measurement below 16Hz. In the DIY community innumerable applications can reach below that. Even the SV Sound or Funk Audio subs can do it with sufficient authority to warrant measuring down to 10Hz or even 5Hz.
I respectfully disagree with you regarding XTZ bring the easiest. IMHO, REW, USB Mic, HDMI audio to receiver/pre-pro is the easiest. It also happens to be the cheapest. The only problem is that the USB mics have a noise floor around 40dB and this could impede some measurements. Needless to say, if one doesn't have HDMI, this is not an option.
The statement regarding XTZ requiring the least up front effort and understanding the audio concepts, prior to taking a measurement, is a bit silly considering the advanced nature of the questions that Omnimic, Holm Impulse, REW, SoundEasy, XTZ, et al, are the answering.
All things said, I really liked reading your writeup. This is the first AH article in a while that I read through entirely without skipping anything.