No Case, No Cover or ScreenProtector on mine. I carry mine in a camera case. The Great Thing about the 521 is the back cover comes off. Easy to pop off and wash off with a little water. Does get a little dirty from time to time, but, it usually just wipes off. Probably easier to keep clean than the glossy, has for scratches would be hard to notice unless they are really deep.
I owned my 521 for a week before I bought a silicone rubber type cover. The raw phone was just too slippery for me. I dropped it 3 times before I got the cover. Fortunately, it was usually on the carpet, so no damage. Haven't dropped it since I got the cover. I cant really speak to scratches, since I don't have any that I know of (yet?)
I dropped mine with a wallet type case on- the kind opened at the top and bottom. It hit perfectly on a hard surface to dent the white back cover at the corner, and take the cover off, there is a black plastic rim all around the front face of the phone that the dent continued into. Acted like a shock absorber and prevented the glass from cracking. A little manual mushing it back flat, and the black rim looks almost normal, but the white back has a permanent dent into the corner. I have since added a tpu inside the wallet case for added protection.
I decide to buy a black rubberized hard case for my 521. It the same case as my white that got dirty all the time and could not be cleaned very easily. Ending up using paint on it to over the spots. The black looks much better and from the front side; shows complete blackness. Very nice indeed!