http://http2.github.io/http2-spec/#FrameHeader section on Flags being unset

"""
The remaining flags can be assigned semantics specific to the indicated
frame type. Flags that have no defined semantics for a particular frame
type MUST be ignored, and MUST be left unset (0) when sending.
"""

Is there a reason that it MUST be left unset? Why not allow extensibility
like we do for unknown frames? I actually don't think there's much value to
specifying behavior for unused bits.