Nobody says that you have to rush through along with the person rushing through unless your the only one taking your time in it then you wouldn't have a choice but by the time you and a friend clear the strike person 3 will be fighting the boss still. Unless he/she has a badass arsenal of gear to kill the boss in under 5 minutes it doesn't hurt any thing but the rusher missing out on the mob drops. Most every one I see rushing are 27+ in level and don't need to kill every thing on sight.