BSD sockets has somehow made it's way through the entire computing world and we're never going to get rid of it. Even the tiniest of embedded systems have decides that sockets are the one-true-way of network programming.
This means that for example, Plan 9 applications that do networking are not tied to a specific network stack, you can mount multiple network stacks concurrently, you can have 'virtual' network stacks (for example running in user space and proxying to a remote host, or doing other neat tricks), and the apps don't need to care.
Also when Ipv6 was added to the Plan 9 stack, no application code had to be modified, because the API nicely abstracts network addresses.
