Ok, I have to chime in here. I dont think TPK's are actually good in almost any case. Why do I say this? Because I play to have a story. A story doesnt make much sense or hold much appeal if the Protagonists are all wiped out.
I guess I have to wonder how much of a story Star Wars (Episode IV) would have been if instead of outrunning the Star Destroyer, Han, Chewie, Obi Wan, Luke, r2D2, and C3P0 would have been blown away by a group of Tie Fighters. What happens then? Some other group with a Jedi, and some droids, and smugglers take up the quest? How many cycles of protagonists can you kill and still keep people engaged?
I get that some people like that DM vs PCs angle. Or like to play the game where you have to be uber cautious about everything you do, the rogue must search every 10' before anyone walks on it otherwise a trap might TPK the party.
I personally just dont get enjoyment out of those styles of games. If a TPK is possible, for me it better be story related and the PCs better have a clear idea that no one might make it out alive. Its one thing when you finally get to the BBEG and you know it is powerful and will have to use every advantage possible....But 12d10 vs low level characters? Come on.
~Desh