In reply to BoulderGoat:
I think the bottom line is that static rope is cheap and tough, so if you do a lot of top/bottom roping then why use your good dynamic rope? The static rope doesn't have the stretch problem meaning it's "safer" for top roping. That is if a big guy falls on your dynamic rope he might hurt himself a bit.
In reality I have set up plenty of bottom ropes using a 50m dynamic rope and some slings to extend the crabs over the edge and it works fine. They have mainly been for our kids rather than adults but even for an adult with the stretch it's safer than leading the route.