Great if you're visiting Barcelona, but how much does it help if you're visiting Rome?
Every time I see this bit, I'm reminded of a bit from the Terry Pratchett novel Moving Pictures:
“Reality is not digital, an on-off state, but analog. Something gradual. In other words, reality is a quality that things possess in the same way that they possess, say, weight. Some people are more real than others, for example. It has been estimated that there are only about five hundred real people on any given planet, which is why they keep unexpectedly running into one another all the time.”
But anyway. While it's not required, in D&D, for the characters to be super-duper special or world-recognized heroes, the game is, in fact, about them. It's their stories, which they are in the process of writing through play. (If the PCs are just there to be spectators to the
DM's story, then the DM should just go write a book. I didn't sign up for
that.)
So I have no problem occasionally saying that yes, they meet a sailor they knew in another port; or yes, they meet a shady caravan driver they knew on on the far end of their newly-expanded caravan route. Neither of those concepts is ridiculous or illogical. And if it
were, well, I've already given lots of examples of how I can handle it.
And if the PCs are actually on another plane or somewhere else where it beggars the imagination of how they could know someone there? Well, I'll deal with that
if it comes up. I don't need hard and fast rules about that vanishingly rare case ahead of time.