Guess which Guy will have a better body in 4 weeks. You better believe its Guy B. So in this case, Guy A is wasting some of his time on cardio.
Okay, so I've kicked cardio while it's down...but when is cardio not a waste of time?
i) When you are getting great results, injury-free, and you're life is not suffering because of the extra time you put into cardio. This would be a perfect world situation. Guess what, we don't live in a perfect world. But if this is you, you're a lucky guy.
ii) It's not a waste of time if you enjoy doing it more than anything else you could be doing in that period of time. Hey, for some guys its a hobby. That's cool. Enjoy.