I intimated above that I like both sorts of games (premised and premiseless).  That distinction led down a trail of “trad” vs “modern”, but I think that’s kind of a false division.  I think what I’m really talking about is more like “campaign play vs quick play”.  For the time-limited, one-shot or quick play scenario, premises are invaluable, because, you know, time.

But for campaign play, I prefer to let premises arise organically, and I think this is the expectation of designers like Monte Cook as well.  I think this can be seen most easily in old-school “trad” games like AD&D and Iron Crown products.  Campaign play invites taking a “modular” approach to worldbuilding.  In such a case, it behooves the GM (and the designer) to simply let the mechanics be the mechanics (i.e. premiseless rulebooks). 

Like with DayTrippers and the MCG games in question, the designer assumes that settings and premises will come from a combination of Modules and GM Prep.  So the rules can just be the rules, and then the GM pulls a world together that suits their style and their players’ interests – some modules and some prep – and that’s where individual premises can be found.