While I personally hate the idea of a screen protector, I must admit that it protected my 5S from a substantial amount of scratches... Every few months I look at the screen protector and it's all scratched up, so I just replace it... It's love/hate relationship since one doesn't wish to disturb the beautiful Retina Screen, but again... if you put your phone in your pocket, you might as well protect it.
I for one am considering a flip-case, if I could find a good one. Then maybe I won't need a screen protector. However, for the 6 Plus, I am seriously considering the Zagg Glass.