I think a fair few of us don't bother with MS unless we are using a 5% category. For cards where the reward is cash equivalent rather than a pseudo-currency like miles, someone is taking a loss on our purchases somewhere in the chain.
We are paying 3-6 dollars per 500 dollars of MS and a month or so later getting 25 dollars as a reward. That 20ish dollars has to be coming from someones bottom line.