Most likely, I turn them on personally. Mostly because to make "PHP" files I make textfiles then rename the extension. Otherwise I'd leave them off.
If that's the only reason you enable that feature, you're obviously not considering all of the other advantages it entails.
Agreed. From a security perspective, it's one of the dumbest decisions I've seen. And that's saying a lot. But making sure that people have NO idea what type of file they're running can cause nothing but bad things.
I tell my family members, "don't open anything that ends with .exe, .pif, [etc]", and my grandpa's friend even has it on a sticky note on his monitor. But when it's sitting on his desktop, he doesn't know what type of file it is. And he doesn't know how to find out. So especially for people like him, having extensions disabled is asinine.