Hi FF,
I have not implemented multiple specifications so far. So i can only tell why I did not consider this, not if the users/I like using it or not ;-)
What I see as the biggest issue is the copy thing of characteristics. I am a big fan of centrally maintained (reference) MICs, and SAP is already not good on this with forceing one to use at least a dummy plant for MICs. As you write reference characteristics cannot used with multiple specs, which makes it hard to maintain the MICs then for the multiple specs.
The engineering workbench as a must for maintainence is a greater issue for users that are familiar with the "old" maintenance transactions. From my experience it is similar to the ME21 vs. ME21n switch. It is always difficult to make that transition for users, a good time I found is a release upgrade/system patch, where we just removed the "old" transactions completely from authorization roles.
Generally I feel that multiple specs would be great for usage in a global company environment, where you have the same MICs, but e.g. due to different measurement equipments might have different tolerances. Up to now I have to say that most users already struggle with implementing/understanding/using the basic elements in SAP (MICs, mat specs, inspection plans, sampling strategies), so multiple specs would definitly be too complicated to explain/keep up running at the moment.
Regards
MH