Mr Yamaha must have thought there was an issue to spend out on new parts for the 7-plate clutch although I am not aware of owners having issues with the 6-plater. I used to rag my 6-plater on enduros and the like and never had a problem.
The biggest benefit I can see from the 7-plate clutch is that the modified cover allows an extra 100cc (or 10%) oil capacity