Some people cheat, yes, like with the flying back in Thornwind CM. Most people that use client modifications use it for GCD which compensates for ping issues to squeeze out maybe less than 10 percent extra dps, and things like using XMLs to rebind skills to different keys. If they're going after people who modify to cheat the people who use it for QOL willl inevitably get caught in the crossfire.
Remember the disaster that was the ban waves when people were abusing the ET skill to gank people? Yeah miss me with that.