OK, the plot thickens. I've been examining the source code to pg_basebackup written by Magnus Hagander. It definitely shows that if you do not issue a "-X f" or "-X s" then the pg_xlogs will NOT be included in the backup. This leads me to a couple more questions:
1) Does anyone know why anything in a WAL segment is required for a database to start? I ask this as the backup_label generated by pg_basebackup indicates a segment where the latest checkpoint is contained. Surely, there has to be some better way to permit the DB to sanely initialize. (Note, I haven't yet examined the sources as to how postgres starts.)
2) If a WAL segment is in fact required for the backed up DB to start, why would pg_basebackup not include those by default? To not do so, doesn't create a backup file, just in this case, a tarball that's worthless. After all, an non-restorable backup, isn't a backup at all.