You're missing the point. Using Giteau as a backup 9,10,12 allows room for another forward on the bench, which we need. Our backs are fit and thankfully not injury prone (exception of Barnes) so ideally Giteau would not even need to step on the field. But why have a scrumhalf and an inside back on the bench if neither go on, rather have only one taking a seat and use the extra forward. Logic, he may not be world class number 9, but we need mobile/impact forwards off the bench, and he has experience that I would trust him coming on with 10mins to go against the AB's, more so than Burgess or Phipps anyway.
Play McCabe as #22, that way if Giteau has to go on for Genia and Barnes/AAC (Adam Ashley-Cooper)/Ioane/Mitchell/Beale go down, you have a utility back. Only problem is if we lose both Genia and Cooper in the one game, Gits would have to go to 9, JOC (James O'Connor) to 10, McCabe 12, AAC (Adam Ashley-Cooper) 13, Ioane 14, Beale 15