Speed isn't a useful tactical defense. The numbers are weighted so a beam warship doesn't sacrifice much to be able to shoot down fast ships. Versus missiles your only effective defense is an anti-missile net (preferably made of anti-missile-missiles AND anti-missile-beams). The lower chance of getting hit is less important against missiles than the extra launcher(s) would be on your PD ships (since you're giving up launchers by mounting more engines). ((Exception, if you're getting targeted by far more missiles than you can shoot down the miss chance might be more helpful. You'd still end up as scrap if that many missiles are getting through, though.))
However, speed is a very useful strategic defense. Being able to choose whether or not you'll actually be fighting against that enemy fleet is enormously helpful. Of course, the enemy fleet might force the matter by going after your colonies, and then your speed is only helpful if you out range them, too.
