Except that the throat culture takes 48 hours to come back, plus the time to get the patient started on the abx after that, on an infection that has a 5 day course to begin with, which got into your office somewhere on day 2. You wait 48+ hours on the backup culture, and you've missed your window to do anything with it.

...which is a big part of why it used to be common, and isn't now.