Like @nikipedia, I’ve never had access to a grade that I didn’t give. At least, not as far as I know. Maybe I just haven’t tried hard enough. That said, I wouldn’t look at a student’s grades even if I could.
First, the only thing that matters for my evaluation is how well a student does in my class. One of my majors in college was music. I was surrounded by people who were extremely talented, but who were often less successful in more academic subjects like history or mathematics. I would hate to think what would happen if their evaluation in one area depended on their evaluation in another, and I rather think that our academic successes and failures are ours to share at our discretion.
Second, I am quite aware of the fact that how well a student has done so far in my class can affect how I evaluate a piece of work that is in front of me. The effect is not severe: I tend to give the benefit of the doubt to students who have been performing well more often than to students who have not been performing well. But a strong student can slip, and a weak student can shine. I’d rather not have anything get in the way of my recognizing those instances, and knowledge of past grades is exactly the sort of thing that might do so.