Tony Vargas
Legend
I suspect it also relates to pace of publication and controlling development effort. If you only have to make a new game element functional with the elements in it's own supplement and those in the PH, that's a /lot/ easier than trying to make it play nice with a whole installed-base of 5e canon (even at 5e's slow pace of release).Core+1 is a ‘rule’, only in the sense of Adventure League and so on make it a rule. So, many home games wont follow it.
However, Core+1 seems a ‘policy’ that WotC implements for 100% of official D&D products. The implementation seems to relate to quality control and republication of content. In this sense, every home game is affected by it, whether they subscribe to it or not.
So it's up to the DM to decide: stick to Core +1 (or better yet, PH-only with feats & MCing turned off), or 'void the warranty' and take responsibility for the state of his own campaign?
And that's just yet more DM Empowerment.