You can knowledgably vote by reading the posts and deciding on their own merit which you choose to ++ or --.
Since I'm asking the question, I presumably don't know which answer is better (I didn't know any answer, after all). I could try both, and then decide on my own, but that takes a lot of work when we have a perfectly good indicator of what everyone else thinks is best.
I'm not suggesting that rep totals should determine my vote on a matter. I'm suggesting that you can't (or shouldn't) always vote on a post, but knowing the rep would be valuable.
Update: Sorting based on reps isn't always the answer, since the posts might be in different threads (questions have been asked more than once, or perhaps the questions differ slightly). This is why
I like the idea of giving up your ability to vote on a post to be able to see it's rep (the only time I won't vote on a post is when I have no -- or ++ opinion of it, or if I don't know if it is accurate)