My take is that "everything in moderation" is a mental crutch that people adopt when there's some part of their diet that they know isn't _good_ but that they don't want to give up completely. It is true that we can sneak in unhealthy food here and there without hugely detrimental effects, but that doesn't mean it's a good baseline practice.

The one scenario where it might be useful advice is with somebody whose diet is terrible, and you want to ease them towards a somewhat nutritionally positive diet; even in this circumstance, a more direct approach of "eat less crap and more good stuff" would be more accurate.