archive_command and streaming replication

PostgreSQL 9.1.2

to keep the cpu- and io-load low, we use
streaming replication with only one slave.
http://wiki.postgresql.org/wiki/Binary_Replication_Tutorial#Streaming_Replication

the slave is not using the "archive_command"
while in recovering mode.
is there any tool to save the pg_xlog-files or
are there other reasons to use the
archive_command at the master?