5 start screws help for a few of reasons -- they're more efficient than single start, so for the same power in you get more speed/thrust out. They also require less rotational speed for the same linear speed, which has two benefits. First of all, it allows stepper motors to operate in a lower rotational speed range where they are more efficient. Secondly, the point at which longer screws start to whip is a function of rotational speed, so with multi-start screws, you can get more linear speed before screw whip cuts you off.
Ahren
www.cncrouterparts.com