"Default" Math Vs "Legacy"?

I'm trying to follow this conversation to the best of my ability. Personally I tend to use legacy. Part of that reason is because of past issues with Xact measure, part of it is the varying opinions on its accuracy, and part of it is not having the depth of knowledge necessary to understand what it's doing exactly, excuse the pun. It can make a complicated alignment structure easy, and save time, so I do use it on occasion, but it's extremely rare. I'll usually provide both methods, and label one as a reference (usually Xact measure), only so operators don't get confused as to which one to base their moves from.

I have to be honest here for the sake of truth. I almost strictly use legacy myself. Probably for the same reasons as most other people (like you) that like to use legacy. I'm used to it, I'm knowledgeable enough to build alignments correct to the standard (with the obvious complicated caveat), and it hasn't changed much in years. I know it's solid.
The times that I will use GeoTol are when I have Datum bonus. I know that you can technically do this in legacy also, but it isn't done correctly and GeoTol/Xact measure gave much more information to tell you how the deviation is zero, when legacy doesn't. This information in critical to test if the code is working properly. If there is Datum bonus and simultaneous requirement applies, I do not give Datum bonus and revert to legacy because I have had less than ideal results from this in the past with Xact, and with testing GeoTol I'm unsure if the results are better. My theory is, if you have 10 different dimensions requiring simultaneous evaluation, and you need that bonus to pass the part, then there is a risk of failure/rejection anyways. Never had an issue.
I do report Datum bonus using GeoTol as non-reference and a copy of that dimension in legacy without bonus as reference so the machinists know what to move and so the Customer can see the difference. This is necessary.
I've tested a few different callouts in legacy and geotol with Koten Smartprofile software (which touts themselves as being certified to the standard somehow) and depending on the callout, legacy can get pretty close. Close enough that I'm not too concerned with moving from legacy completely. But I got much much closer results with Geotol when Datum bonus was applied.
This all rides on us trusting any software we use, including PC DMIS and legacy.

But, I'm sure we can all agree that bonus modifiers on threaded holes are useless.

Bonus on threaded holes are inconvenient, as an inspector, but if the designer wants to predict if a fastener can get started, rather than how it ends up in final torqued state, an MMC on threads is far from useless.

Bonus on threaded holes are inconvenient, as an inspector, but if the designer wants to predict if a fastener can get started, rather than how it ends up in final torqued state, an MMC on threads is far from useless.