I agree that the true PD never happens in human existence, and that's yet another reason why I'm outraged at using a mathematically flawed decision theory to teach incoming students of rationality that they ought to betray their friends. (C, C) FTW!

(Actually, that would make a nice button.)

But I defend the use of simple models for the sake of understanding problems with mathematical clarity; if you can't model simple hypothetical things correctly, how does it help to try to model complex real things correctly first? In real life, no one is an economic agent; in real life, no laws except basic physics and theorems therefrom have universal force; in real life, an asteroid can always strike at any time; in real life, we can never use Bayesian reasoning... but knowing a bit of math still helps, even if it never applies perfectly above the level of quarks.