So, the conclusion to this is that these new mechanisms will improve UML in a number of ways.
Exposing address spaces to UML will make it cleaner and more efficient. By reducing the number of processes on the host, it will reduce the footprint on the host. By allowing the UML kernel to exist in its own address space, it will make it practical for 'jail' mode to be the default, improving the security of UML.
A faster system call interception mechanism will improve the performance of UML. By eliminating the constant context switching that is currently required, this will reduce the load of a UML on the host, improving its hosting capacity.
Finally, some ways for the host and UML to communicate about memory management will allow them to use the host's memory more efficiently, again improving the capacity of the host.