PostgreSQL Binary Replication -do i really need to copy pg_xlog separated?
It works fine, until it doesn't.  Then you get a backup which won't start, will start but won't connect the master to stream new data, or perhaps one which will start but is silently corrupted (unlikely, but I wouldn't rule it out). Have you tested this method for a very large server under very heavy load?

If you use pg_basebackup, it will either take care of it for you, or will notify you that the backup failed before you discover that the hard way, or notify you that you need to have a separate WAL archive.  Depending on which version of PostgreSQL and which options you give to pg_basebackup.