Server-side base backup: why superuser, not pg_write_server_files?

Hi Hackers,

I just noticed that the new server-side base backup feature requires
superuser privileges (which is only documented in the pg_basebackup
manual, not in the streaming replication protocol specification).

Isn't this the kind of thing the pg_write_server_files role was created
for, so that it can be delegated to a non-superuser?

- ilmari