Yeah, I'm not getting this argument either. Or the guy who asked why have any credit cards at all if you're not planning to go deep into debt.
I've got an AmEx Blue Cash Preferred that pays 6% on groceries. My Capital One Savor card pays 4% on dining and entertainment. I've got Chase Freedom and Discover it cards that pay 5% on rotating categories, and a Bank of America Cash Rewards Preferred card that allows me to pick my own 3% category, including for online purchases, so I'm pretty much covered on everything at 3% to 6% cash back.
I can walk and chew gum at the same time, too!