Analysis Services Security - SQL Server Analysis Services 2012 Cube Development - page 261

If you use the preceding expression to define read-contingent permission (as
shown in the preceding screenshot), the role members will see #N/A for the
calculated measure Average Sales Amount because you did not grant
them the permission to see the Order Count measure that is necessary for
calculating the average. On the other hand, if you enter the same expression
under the Enable read permissions checkbox, role members will see the
calculated measure even though they won't see the Order Count measure.
There's more...
In some cases, when you must use multiple fact tables, you have a choice of creating
multiple cubes or a single cube with multiple measure groups. If you use a Secur-
ity measure group, you should create a single cube with multiple measure groups
because you would only have to process the Security measure group once. This
is presuming that the same Security measure group can be used for securing di-
mension data for all other measure groups which may or may not always be the
case.