It is definitely not required or even necessary on the retina MBP. As the screen is made of glass, it's pretty difficult to scratch, but it does happen.
I edit photographs most of the day and the reflection and glare from the glossy screen interferes with that process for me.
The particular screen protector doesn't reduce the quality of the screen for me at all. It also does not produce any funny effects when I view the screen from different angles. The one I have adheres to the entire surface of the screen, so there is not gap between the screen and the protector. There are some models that only stick to the edges of the screen and the viewable area isn't directly on the surface of the screen - I imagine there is some small gap, but with that, it would clause some strange viewing effects.
This is the one I have...
http://www.amazon.com/Kuzy-Anti-Glar.../dp/B00C2CRDJU
I only recently applied it and I will likely never remove it for the life of owning this machine. I don't touch the screen or let other people touch it so I don't think it will get scratched. I imagine if you use a sharp object carelessly to remove it, you could damage the screen. I've had other screen protectors on my phones and I've removed them and reapplied them and they've look just as good the second time around.
However, the protector on the retina is so large it's probably not a good thing to remove it and reapply it. It's not that easy to apply in the first place. It takes some patience and skill to apply it so that it's perfectly aligned and there are no air bubbles or dust underneath.