Hi Mattias,
you're right "never" is too strong. The reason isn't related with the performance (ok several *COMMIT in default logic will cause performance issues) but with the behaviour, when you send data from a data entry, etc., after the default logic is triggered and will receive the scope from the changed values as a combination of the whole member values of all the dimensions.
If you use an *XDIM_MEMBERSET (or *XDIM_ADDMEMBERSET) in the default logic you will change the scope, so it rare to be used.
*XDIM_FILTER like *WHEN just filter and doesn't redefine the scope.
Regards
Roberto