Talk to your lawyer. It may not necessarily be a win to add that clause. You have the ambient right to fire someone (in almost every locale in the US) with no reason whatsoever. Why qualify it?
Actually I was joking about adding. I have no plans to add such a thing. Our interview process has fortunately been solid enough that no one we've hired has been less than solid. And I think such a clause would backfire, since many of our best devs wouldn't have accepted on such conditions.
