Originally Posted by Cadfael

In addition to the above...

Many cyclist remove this. It can look a bit tacky on a bike that is well looked after and well maintained... a cheap bit of placky just sitting there doing nothing it would seem but getting full of oil and crud, but it does serve a purpose. If you are absolutely sure your read RD is set up correctly you could take the chance and remove it, the rear cluster DOES look better for the lack of it in my opinion. It was a no brainer for my bike... I never ever go to the largest ring on the back cluster. But at the same time something could damage your RD which could send the chain over the cluster.

Personally, I like the large metal ones you see on some cheep bikes with slots cut into them.