Streaming rep - why log shipping is necessary?

Hello,
I was reading the SR docs, and have the following question:
Is there a fundamental reason why archive_command etc. is required in
streaming replication mode?

Can`t setting up the standby be more like:
pg_start_streaming_backup() on the master (this will be queuing up
files in pg_xlog)
copy the data dir
set up the slave to connect to the master via streaming protocol
set up the master to allow connections from the slave
start slave (slave pulls the necessary WAL records from the master via
streaming, and signals the master that it`s done backing up)
When standby starts accepting connections, we know that the standby is OK.

archive_command, restore_command, etc. would be configured empty in this mode.

The failure mode for this is the pg_xlog directory filling up on the
master before the backup is done. But then, we can tell people to use
the more combersome, current setup.

Greetings
Marcin Mańk