This would just make things more complex. People will get more or less credit depending on how others vote. Think of it like this: if one answer is great and one answer is "half good" as you put it, someone might upvote the half good answer, but others won't. But they'll all vote up the great answer.
Remember, if an answer is okay but not good, you can add a comment to say what's wrong with it. If an answer is incorrect then vote it down as normal.