I found this "Treatment with erythromycin does not alter the...

I found this "Treatment with erythromycin does not alter the course of disease, but reduces the infectious period to 5 to 10 days." and this

"Although B pertussis is susceptible in vitro to several antibiotics, such as tetracycline, erythromycin, and chloramphenicol, the efficacy of these drugs in patients during the paroxysmal phase is not convincing. "

http://www.ncbi.nlm.nih.gov/books/NBK7813/

Reducing the infections period does not equal killing the organism.

Basically antibiotics are administered because they don't know what else to do. In my opinion.