I don't need screen protector because I am not going to touch retina display and no one will touch my rMBP! and I have keyboard cover but never use it until I go to school.
I just bought Speck SeeThru hard case last week and it looks bit thick to me I wish I could have bought Speck Satin case.
I have some colleagues who seem to drop their computers fairly frequently and as a result have chips and dents on their systems. A hard case would probably be a very wise investment for them. I have never dropped my system and am very careful when moving it around, thus a hard case would be a waste of money for me. There were also some reports of cases raising the computer's temperatures by interfering with the passive cooling from the casing, which could be important if you run your system hard.
The keyboard protector is similar. Do you often eat or drink around your computer? A keyboard protector could help to guard against crumbs and minor spills, but otherwise it's a bit extraneous.