CK,
allow me to take a stab at your rhetorical question.
First, I congratulate you for asking objective and challenging questions and indirectly stating the obvious....
I think vocalist trust that the front of house sound is accurate and monitors are...well...just....MONITORS. However, many guitarist can't play without the tone and touch sensitivity of THE amp right at their feet....or within immediate sonic range....up close and personal. This most likely comes from sitting in a practice room right in front of the amp. That sound is not available anywhere else, certainly not to the audience... Large venues bands and big name acts don't have an amp at their feet or within that direct immediate range. They have monitors, earbuds (IEMs) or massive slaved backlines that don't sound like THE amp in there favorite setting. They are professionals and have left behind the illusion that there ideal "in the room" sound is something the audience will ever hear. They learn to play listening to the tone the audience hears....the same tone the KPA captures so accurately, the mic'd amp and cab.
So my advice.....simplify your life and go pro, learn to play with the feel of a mic'd amp through FRFR amplification. Its different but hey, its what your audience hears. If that's not good enough, improve the overall tone in the front of house until it meets your standards. That way the audience can enjoy it too!
This isn't meant to draw flames, just to state that modelling, and now profiling, really puts us, the amateur guitarist, in the same setting as the big boys. Just my humble opinion. If amps and cabs are your thing, enjoy!
bd