They might, if we ever get to the point (many years into the future) where fiat becomes obsolete.  The advantage of credit cards is, at least in part, for people living paycheck to paycheck, they can spend money that they don't have yet and pay the bill later when they've been paid.  But I agree with your overall point, it does seem rather counter-intuitive that someone would use both a credit card and bitcoins at the same time.  Debit cards would make more sense, though.