Think of Linux on a box with not a lot of memory but a big, fast swap-disk. On any ordinary CPU, you might curse this situation as you listen to those disk drives chatter. On a mainframe, its more like a goal that you might want to shoot for. Let them do something. I see no reason why something like Linux couldn't eventually be tuned to work well in this kind of environment, with the kind of throughput and scheduling efficiency that MVS is famed for.
Some people talk about capabilities as a means of doing security.
Others talk about how a rogue kernel module can do a lot of damage to the kernel proper, like crash the machine, for instance, or maybe do far more malicious and pernicious things. Now imagine a hardware architecture that could protect against that. So instead of having user space, with limited privileges, and kernel space, with full privileges, you can have multiple spaces, each with full privileges, yet each protected from the others. You don't want a rogue X client trashing other clients or even taking the X server down. The price for this safety is high.