Well, it certainly doesn't need one; it's a matter of personal preference. My own experience was dreadful. As soon as I put it on (the Mini), it was full of bubbles. Of course, this was probably due to my carelessness/inexperience, but I immediately took it off and threw it away. I just live with the fingerprints and clean my screens regularly with a microfibre cloth.
Another point made by my brother-in-law...if you use a screen protector, at the end of your device's life (when you might want to sell it), you'll have a pristine, like new screen, and an obviously duller and used casing. Mind you, I'm not endorsing his view in any way...just throwing it out there. If one is reasonably careful and cleans the screen(s) regularly, it should be just fine.