Interestingly, even now that many merchants are allowed to offer a discount for using cash, most don't because they've likely figured out (a) if they lose just 1 sale in 30 it's likely a net negative for them, plus (b) people are likely to spend more, especially on impulse purchases, when using a card, and (c) there is just a sense that it can be annoying to customers to have to pay more for using a card, taking away from customer goodwill.
I imagine it's because of lack of enforcement?
So, I still favor the discover (despite now being part of Morgan Stanly?) for most purchases, but when its rejected I fall back on a MC with a cashback program.
That program pays for my computer hardware...