This is kind of like Relative paths and Full in URL's. I will think about it a bit and comment.
I am always explicit in server connections and don't use default servers (or default PI system, default database in AF). My vote would go for "insist they name the server".
Besides from a debugging point of view surely you want to know where your data is coming from rather than rely on the PISDK default server configuration on the PI WS/DS server.
There is a use case for partial tag names... An application can easily be moved from the test PI Server to the production PI Server by simply changing the default server. Many customers take advantage of this functionality in the PI Data Access products. For this reason (and for consistency) we should support it with PI Web Services too.