David,
FTWRL is only needed to copy other files like *.FRM or MYI/MYD files if you have MyISAM. Because you cannot take a consistent snapshot of tables other than InnoDB without FTWRL this is often necessary.
On the other hand, if you are absolutely sure of 2 things 1) No DDL occurs on the tables being backed up at the time 2) You only have InnoDB tables aside from the mysql schema, and if you do have MyISAM (nothing will ever write to it during backup), you can use the --no-lock option with innobackupex to avoid the FTWRL.
So these are not necessarily contradicting statements