Welcome to FlyerTalk, gbahri.

Depending on where you want to fly, only one (or two) of the three major airline alliances may have award flights. So many people don't want to restrict their miles collection to just one airline / alliance. Thus some people might get Amex MR for Delta, while using Chase UR for United, for example, so that they'll have (the ability to quickly transfer to) miles whether only Delta shows an award or only United shows an award.

On the other hand, if you don't have a plan to collect enough points/miles in each program to get to a "usable" level (for whatever you want to do with them), then yes it can "hurt" you to have split your collection of points/miles to too many places that aren't combinable.

And thus there's no one answer (as to whether to collect multiple currencies or just one) that fits everyone.

As to your specific question, yes, if you buy groceries, then definitely either Everyday Preferred or the no-annual-fee Everyday would make perfect sense to pair with any other Amex MR card. Whether you should go for the Everyday Preferred or the no-annual-fee Everyday depends on if you'll use it enough to make the extra earnings pay for the annual fee. (That, in turn, is a balance between how much you spend on credit cards annually and how much you'll be applying for other cards where you have to do big spend to get a signup bonus. Big spend for a signup bonus may mean that for a while you have to stop using other cards, meaning you no longer get as much justification toward annual fee on those other cards.)