Indeed... few reasons I don't find this is necessary
1) It makes hand holding the camera uncomfortable, why do you think the manufacturer make those rubber grips at hand held areas are for ?
2) You bought a nice black and sleek camera, and you destroy the look of it with those third-party rubber skin ? Man... I cannot believe when I see people wrapping up their iphone and ipod ? Why even bother a sleek chrome finish device and you wrap up with those rubber thingie..
3) Dust still gets in, water still gets in, You find it will be cleaner ? Trust me on the contrary, it wouldn't because if your hands never touch those areas under the rubber skin, dust just keep building up there. Your daily handing will help to wipe the dust away. The casing is build to last, it is not made of paper.
4) If you drop it, you certainly is not expect G-shock right ? You either burst your lens or you traumatize the electronics and mirrors and glass/prism inside, the casing is the last thing to even consider about.
This silicon casing can trap moisture inbetween your gadget and the silicon. Yes it can protect your hard earned camera from minor bruises & scratches but remember one of our major fear is moisture going inside our beloved gadgets. Unless you have the free time to wipe your cam after a photoshoot.
Actually it depends on individual lah. I have been using it for the body to prevent it from bumping to hard objects when moving around.... I usually sling my cammy around... Anyway, I got it as a free gift bundle when I purchase the cam, so might as well use it.... So far so good lor.. I have big hands so I feel the grip still alright. As for trap moisture, have not notice it... its not really tightly fitted (loose contact between body and case) on the cam, so air still flows true and I guess, moisture suit not be much of an issues.