Throughout all of Yoreh Deah Chelek Alef and Hilchos Pesach (and perhaps other places in Shulchan Aruch as well) we see the idea of "blias" ("absorptions") by celim. That is to say there is a "taste" that is left in a pot or pan after cooking in it. Therefore if someone would cook meat in a pot, then within the same day cook milk in it (even though there is no meat in the pot now), still there is a "taste" of meat that was left inside of the pot walls when it was first cooked. That taste will now come out of the pot when cooking milk in it and will prohibit the milk.
This is all true of a metal pot. However, a glass pot, according to the Machaber, doesn't take in any taste at all. On the other side of things, if one has a pot made of "cheres" (earthenware), he would never be able to expunge taste from it once it was cooked in (not like a metal pot which one could kasher by making ha'galeh on it).
I'm having a difficult time understanding how this din (rule) is still relevant to today's pots. Is there really a taste left inside of a metal pot? Once upon a time a metal pot that was cooked in over time would slowly become dirty, grimy, filthy, covered in black, clearly used. One could look at this and understand that there is a "taste" left in the walls of the pot. However, I don't see how this is true of a normal metal pot in today's market, which is smooth, finished off, and more or less retains it's original color and look and stays clean over time. Where is the "taste" in the walls of this pot? If someone would cook milk or meat in it and then cook water in it, would the water have any taste to it (assuming the pot was cleaned in between)? While I don't see any posek (halachic decisor) using this argument even as a "tziruf" (argument in combination with other leniencies) to be lenient in a question of issur v'heter (forbidden combinations), still, I'm looking for any poskim that perhaps spoke about this or entertained the possibility. Maybe today's "metal" pots are more like a glass pot, which the poskim say doesn't retain any taste of what's cooked in it.