As a violin luthier, but one whose first instrument (to play) is the guitar, I have also been fascinated with this idea, enough that I have experimented with soundposts in guitars.The idea that a soundpost in a guitar is an uncontested failure is far too broad a statement. Certainly not all guitars are in need or will benefit in some way, but there are some that can.
The first thing to recognize is that placement of the post in a violin is a critical procedure, subject to altering the violin sound with even minute variation in position, quality of contact, or force, and there is no reason to suspect it should be any easier in a guitar. In fact, it will be more difficult to find an optimum position, because there is no obvious zone or location, such as there is with the violin. In other words, just sticking in a post haphazardly in any old place and declaring the result an unmitigated disaster is hardly the last word on the idea.
My general take is that there are indeed a few guitars that might benefit from a soundpost, either to improve the tone, or to cause the guitar to have a different sonic signature. In fact, I have one in a jumbo acoustic at the moment. It strengthens the tone and increases overall volume on this guitar. I have others that do not seem to benefit from a post.
For anyone inclined to experiment, a first principle of positioning would be to put the post on the treble side. It does not need to couple the top and back that much to make a difference. In other words, putting a post under the bridge would likely not have a positive effect, on an acoustic anyway, so think about keeping it closer to the ribs. One only needs to change the guitar's modes a little bit to have a fairly large change in sound. The neat thing is it is completely reversible, and allows one to change the tone fairly easily.
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