snip
better job of conducting heat from the iron tip down into the PCB hole.

That's probably true, but there's also a lot of amalgamation going on.
That's why copper soldering iron tips get pitted from solder; it
amalgamates with the copper.

What a shame lead-free doesn't do the same ... d;~}

Seriously though, most soldering iron tips have been made from nickel plated
iron rather than copper, for many years now, and they get just as pitted as
the copper ones did, but at least you could file the copper ones down. I
never used to put a new bit in my old Adcola, until it was a 'stump'. I am
more of the opinion that tip erosion, both back then and now, is caused
mainly by the mildly corrosive action of the flux in the solder core, and
that the nasty gob of metal that you find on the tip when the iron hasn't
been used for a while, is oxidised components of the original solder
almalgam. Smitty, what thinks you ?

Arfa