Strafe has a nextdelay of 4 which means that a target can be affected again if more than 4 frames have passed. So as far as I understand it, if strafe shoots an arrow once every 2 frames, the first arrow will have an effect on a single target and the following two won't. However, the arrows are usually distributed among several monsters while really dangerous ones like Diablo or some random boss with crazy extra abilities will be killed with something like guided arrow or, in case of a physical immun bosses, magic arrow.
I think it's worth putting on IAS gear for strafe in PvM, but I wouldn't put great efforts into it. For example, if a source of "chance to cast amplify damage" is needed, I would use Atma's Scarab instead of Highlord's Wrath.