Hi all, I think that slow cooking would work just as well with the tin foil option, but as Xtra News mentions there are subtle differences; (apologies if this next bit sounds a bit gross) the porous nature of these pots means that, with fatty foods such as mutton and goat, some of the unwanted fat is taken up by the pot. I havn't had goat for ages, the last time was when an African friend made me a goat curry, delicious!
My mother tells me that during WW2 it was not unknown for people to build slow cookers using a crate and bails of straw, I think that once well heated the pot was placed in the straw and allowed to continue cooking on the residual heat!