I'm with Stig on this one.
When polycarbonate begins to yellow, it has lost it's resilience and impact-resistance properties. If struck by a piece of road debris, a bug, or a bird (it happened to me once on the Honda) not only will you get hit by whatever it is, you'll also get hit with a shotgun-blast of small, sharp, polycarbonate fragments at whatever speed you're riding, plus however fast whatever it was that hit you was going.
A damaged or deteriorated windscreen is dangerous.