No one wants to train new employees. It is expensive.
I totally disagree with the filter. I don't want employees that aren't going to tough it out and quit mid-project either. That isn't what we are talking about though. We are talking about people that probably switched jobs for any number of reasons. Not hiring someone just because they switched jobs or independently consulted, that seems so arbitrary it is almost comical.
It really is a two way street. I worked at a place that had 98% employee retention for multiple years through the boom. Almost everyone there was heavily recruited and almost no one left. Even the people that were previously independent consultants and the ones that had job hopped. Why? It was a great place to work with above average compensation and awesome people to work with.
My point is that if you compensate people well and you provide a great place to work you don't have to filter for potential job hoppers. You are then free to filter for the important things like work ethic, aptitude, skill, and fitting in.