Having buffering doesn't make the stream seekable.  So the question is, is the *design* of the IO module that '+' requires a seekable stream the best behavior, or can that constraint be relaxed?  You have to keep in mind that the IO module is a bunch of building blocks, which are plugged together automatically for the most common scenarios.  The goal is a portable, consistent IO system, not one that completely mimics unix/C IO primitives.