But they can have a chilling effect in attracting investment (been there, done that, in proto-companies where other key people had been foolish enough to sign them).

Part of the calculation is that if you turn out to be successful the threatening party will have every motivation to sue.

I'm certain the unenforceability of them in California has a lot to do with the sustained success of Silicon Valley (although in this case, as a founder, his might be enforceable there).