Kennel cough is the equivalent of the common cold in humans. Actually common colds would be a more appropriate term. There are thousands of strains or varieties of the common cold. The same is true of kennel cough.
The vaccine might protect against one strain, but unlike rabies, parvo and distemper (ok, I know they aren't 100% effective) if your dog has regular contact with other dogs, chances are that he will at some point contract a strain other than the one for which he was vaccinated.
Our vet does not routinely innoculate or inject for kennel cough. I requested it because the local training, day care, boarding facilities require it.
As far as needing it, since it won't protect against all the varieties, what's the use?
Tom