1) We do give Role A and Role B to All Users but using Desktop rules and filter ID properties display either Role A (for Desktop A) and Role B (for Desktop B)
2) The business case in my scenario is different, we have two different desktops (one old and one new) for the same user, We filter the entry points based the desktop filterIDs, but there is no filtering the portal content.
As I said, I've already talked about the need twice. Let me know if you have any other questions.