Due to the fact that pinned and recessed revolvers have long been discontinued, the feature(s) add value to the collectors market.
Pretty good explanation. I would add that there is a general perception that the older pinned & recessed guns were "made better", and so are also desired by shooters outside of their status as discontinued collector variants.
Part of the reason for this is that S&W did have QC issues after they changed their process. There were a number of factors involved, but most folks felt that S&W was "cheaping out". When its done right, either way works fine, other makers never pinned the barrels or recessed the chambers on magnums and their guns are perfectly fine without it.
Pinned & recessed was felt to be a sign of the extra care that S&W took making their guns. Reality not withstanding, when S&W dropped the P&R, and had some less than full quality guns get on the market, the legend became reality, and people pay more for it, both to collect, and to use.
I had a 629 (stainless .44 mag) with the long barrel (8") made shortly after the change (82-83, depending on model) and after a few hundred rounds the barrel visibly canted. Believe me, there is nothing that builds consumer confidence like watching the barrel of their top of the line .44mag UNSCREW!
Oh, they fixed it, no problem. In fact, that's what they said when it came back with barrel on straight, they said they found no problem with the gun. I digress, sorry.
Pinned & recessed guns are worth more to some people, because its what they want.