Currently Linux has the filesystem namespace for mounts which is beginning to prove useful. By adding additional namespaces for process ids, SYS V IPC, the network stack, user ids, and probably others we can, at a trivial cost, extend the UNIX concept and make novel uses of Linux possible. Multiple instances of a namespace simply means that you can have two things with the same name.
For servers the power of computers is growing, and it has become possible for a single server to easily fulfill the tasks of what previously required multiple servers. Hypervisor solutions like Xen are nice but they impose a performance penalty and they do not easily allow resources to be shared between multiple servers.
For clusters application migration and preemption are interesting cases but almost impossibly hard because you cannot restart the application once you have moved it to a new machine, as usually there are resource name conflicts.
For users certain desktop applications interface with the outside world and are large and hard to secure. It would be nice if those applications could be run on their own little world to limit what a security breach could compromise.
Several implementations of this basic idea have been done succsessfully. Now the work is to create a clean implementation that can be merged into the Linux kernel. The discussion has begun on the linux-kernel list and things are slowly progressing.