The only advantage of UP is to support the Papilio Pro, but that port is also very limited in other ways and the Pipistrello provides a more reasonable platform that also supports AMP.
On the other hand, RPCs on UP are difficult to implement with the session.c protocol system (without an operating system or coroutines), along with many other minor difficulties and maintainance issues. Planned features such as watchdogs in the core device are also difficult on UP.
Sebastien Bourdeauducq