Eh... I don't follow his logic on the facing thing. Have to institute spot modifiers? For facing? No, you really don't. A beholder is a creature with eyes facing all directions, so it doesn't need to turn its "head" quickly or anything... on could easily conclude that the beholder could only rotate slowly, thus confining facing considerations to such a creature. No need to extrapolate this to all creatures.
It seems to me like Mearls is falling into a trap I usually associate with, to be frank, ENWorld... that is, when one could make assumptions, making ones that fight against the existing ruleset rather than equally fair assumptions that support it. Yeah, sometimes this can be good for generating campaign ideas, but when it just results in rules instability, it's time to stop, back up, and make the assumption that supports the existing rules.