You're thinking of it that with the cash back you can get let's say $250 cash back on $25,000 worth of purchases, which you can then go ahead and get a cheap flight. But what about the other way around? I just paid 50,000 miles to fly EWR-Paris(CDG), CDG-Venice(VCE), Rome(FCO)-LHR, LHR-EWR. With cash back that would've been $500, but the tickets, had I paid for them, would price out at $3,000 or more.
So it depends how you want to look at it. I would say the miles are more worth it.