3 things come to mind that can cause rear wheel spin (when on center stand) when clutch is adjusted right.
1. we have wet clutchs.. that means that there is oil there... could just be parasitic drag from the oil.
2. warped friction or steel plates... including the pressure plate.
3. deposits on the plates that are causing drag (rubbing adjacent plate).
remember, the purpose of the springs in the clutch are to press all the plates together.. when you pull the lever you open the clutch, and compress the springs. while not a bad idea to replace them occasionally, they wont fix this kinda problem (however, while in there replacing springs you may unknowingly fix another problem that was causing the drag in the first place)