Well i think the real issue is that you can't enumerate the protocals that "use netloc". All protocols are allowed to have a netloc. the smb: protocol certainly does, but it's not in the list.
The core issue is that smb:/foo and smb:///foo are different urls, and should be represented differently when split. The /// form has a netloc, it's just the empty-string. The single-slash form has no netloc, so I propose that urlsplit('smb:/foo') return SplitResult(scheme='smb', netloc=None, path='/foo', query='', fragment='')