I agree, the Harasser is meant to be just what it name states, a harassing combat unit. The low top speed makes the Harasser a simple target that can't circle around even an Atlas without having the guns bared on it. I also vaguely remember the harasser being a bit less...bulky than it is right now, but you guys are saying that MWLL isn't following the canon series's example, so that's up to the game designers.
Simply put: More speed and a more functional turbo.