Lots of arguments in this thread can be boiled down to, "its a boring button to press that should just be baseline macro'd into our abilities" and you could say the same with lots of things in wow. For example Rogue poison, why do I need buttons on which one to apply? Oh in pvp you use wound, so why not just add them all together and forget about it? The cool side of hunters mark comes from pvp where putting this ability on a rogue/druid/hunter can be very very useful and definitely worth the gcd.
Yeah on a single target raid boss you use it and forget it but in most fights youll have to make a choice, do you need this extra 5% dmg at the cost of a gcd? The benefit is you get to make the choice and a good hunter will know when and where to swap hunters mark for the best benefit. Hunters that dont want to be bothered can just leave it on a target all fight and be fine. Its silly how people want to be casual and use 4 buttons while also thinking their play style should sim at the top.
losing a 5% dmg buff brings you from 50k dps to 47.5k, assuming you are constantly attacking on a single target. that dps wont matter in most cases and if it does well you just need to get better to know how to maximize your dmg in a given fight/situation.