No. Empirically, they're as predictive as or more predictive of future educational performance (usually first-year college GPA) than test scores, despite grade inflation (which isn't as extreme as some would have you think).

The advantage of GPA is it's an average over a long time period, unlike a test, which is a slice at a very brief moment. The disadvantage of GPA is variability in course difficulty and grading standards.

What's interesting to think about is how variable grades and courses really are across institutions. For example, colleges do try to coordinate content a bit because of transfer issues etc.