Of course it might happen there was just a bug in fe-exec.c, however that is a smell. Lots of clients might need to update server_version parsing logic for no reason except "support 10devel kind of versions".
1) By saying "just parse server_version" you are basically forcing every postgresql client (except libpq) to implement, test, and support its own parse logic. Do you care of having robust clients that will not break in parts when tested against 10.whatever?
Craig> This means that at least when connecting to newer servers clients no longer have to do any stupid dances around parsing "9.5beta1", "9.4.0mycustompatchedPg"
2) Official documentation suggests "see also server_version_num for a machine-readable version."