Numbers are bad because they're usually quite arbitrary. Upon playing a game, I've never really thought "These graphics deserve an eighty percent, the story a ninety-five, but the controls lose a few points and dip to a sixty-four."
It's difficult to go from a subjective review to an objective number. Also, I find a lot of reviewers seem to work on a scale from six to ten rather than zero to ten. Remember: a grade of 6/10, or 60% is still a passing grade. I don't give that out to a game that annoyed me.
If an example would help you: a lot of reviewers and such have done reviews of Big Rigs: Over the Road Racing. This game is widely touted (for good reason) to be one of the worst games of all time. It's still gotten scores as high as 1/10 and 1/5, despite not even working. One of five maps crashes the game, you can't collide with anything (not even houses), opposing trucks don't even MOVE (a later patch added some AI, but they always stop before finishing the race), you can accelerate infinitely in reverse yet stop on a dime, sometimes victory conditions can be met before you even start... this game deserves NO points, yet it still recieves some.
I don't think numerical scores are bad in and of themselves, it's more the people who use them that make them bad.