I've used a silicone cover similar to what is in the OP on every Kindle since the first version and we now have 5 Kindles total. These sort of covers are great as they do indeed add some bump protection but more importantly they improve the handling of the reader. Well, except for the first Kindle which has a rubber back and being wedge shaped allows a more positive grip if you hold the reader in your left hand.
And it is correct a silicone cover or sleeve ( am not sure what to call actually call them) does not offer any screen protection. Still I would not add any of the clear screen protectors. We all use a-frame style Kindle covers. We have been through all the different style hard covers from Cole-Haan, Amazon, M-edge and a couple others. We found any of the book-style covers awkward to handle and with since the adding of the new hinge it just doesn't feel secure. The a-frame is more is nice because the front flips over the top and just handles better over all. The model we settled on is from M-Edge and the main reason I first tried it was because it didn't have the hinge, it uses a couple, wait here is a link to the cover on Amazon, it is easier than trying to describe it
What I do is usually slip my reader with it's silicone sleeve into the cover then just slip the two elastic corners over the reader's corners to hold the reader in the hard-cover. That was the biggest selling point for me, the reader fits into the hard-cover with the sleeve on the reader. You do not want to keep taking the silicone sleeve on and off as each time it will stretch out of shape a bit more until it won't recover anymore.
But the combination of the sleeve and the hard-cover really seems to be ideal for all the moving about we do with our readers. And none of us ever use a reading light and we have tried the new lighted cover for the K3, well I did since I have a couple K3's for under the tree and needed to try the new lighted cover which I sent back in favor of what we are used to and like.
Oh, if you use the silicone sleeve it puts a lot of strain on the hinge in covers that use a hinge to hold the reader in the cover so I really would not recommend using both in that style hard-cover.