Hi Marco,
Have you gone through this note ?
Hopefully this gives the information why the strategy assignment was removed
498352 - Maintenance plan w/ maint. call object service entry sheet
" The scheduling is executed at maintenance plan level. The reference to the service data, in particular validity start date and validity end date of the purchase order is created in the maintenance item
Only exactly one maintenance item can be assigned to a maintenance plan of this category as a synchronization of these dates is not possible during the maintenance plan call and due to other technical reasons.
The option to assign maintenance plans of the service category to several items was created in Release 4.6 due to a program error within the conversion to the maintenance plan interface. "
Regards
Terence