I think the discussion belongs to MM, at least for the fact that these products are interfaced with MM.
But I am not sure if we find answers. Maybe some of my followers is familiar with one of these products and was involved in a decision process and can share some information when reading my reply here. The Ariba questions in MM forum are usually stay unanswered.
One of the guys who had asked those questions could potentially share some info.
I actually guess you are much closer to somebody who can give you an answer, as you had already another question on Coupa interface, so I believe you are directly in contact with a company who uses this software.
A taxi driver could ask why people are buying cars, all they need is what he has.
Why do companies implement WM and other are satisfied with IM only. Why a SD module to do goods issues with 601, MM module is sufficient as it has 231 to do the same.
Those modules fulfill specific needs which cannot be fulfilled easily by MM. One company has these needs, others don't.