Zero benefit. The weight pushing down equals the weight going up, so there's no effect. I've read one analysis that says you actually LOSE with ankle weights, because the momentum helps carry the pedals over the top.
You might be able to construct a theoretical scenario where you're pedaling clipped in and pulling up hard, so you'd get a little extra workout lifting the weight. But you've still got the opposite weight going down, and many studies have shown that nobody, even top pros, pulls up for more than a few strokes at a time anyway.