Wouldn’t this (unfairly) skew the higher ratings to games with humor?

I don’t think, There is quite a few thing that there isn’t in the rating but would add to the overall expeirence such as story.

Also as IHateATMFees said it would mess up the ratings.

I don’t know. The overall score allows you to put weight on certain things that heavily influenced the game. This wouldn’t be possible by just adding the sub scores. I rated a few very nice games that had no applicable rating in one or more categories, yet the overall score allows me to express that I liked them.

I find having a separate “overall” rating is pretty useful. There’s been several games I’ve voted which might have had significant flaws or shortcomings, yet still show more creativity and potential than the ratings might reflect — “the whole is more than the sum of its parts” and all that.