Trash is there to help pace the dungeon itself. Running long corridors to bosses and only bosses would make a dungeon incredible boring.
That said, since you mentioned WoW, multi-pulls were NEVER the norm and instead are what "crazy" tanks would do. These would be guys overgeared for the instance using cooldowns and trinkets to pull it off. The trash packs were at least designed with a kill order and threat levels in mind. Even the "easy" dungeons in WotLK were like that, with Cata and Mists dungeons following that concept. The only mobs you'd skip was stuff not directly in your path. The side mobs in Pit of Saron come to mind. The packs in the other platforms in Halls of Lightning also come to mind.
Instead what we have here is sac pulls and lockout abuse. Both scream "not-intended".
Your reason for killing trash: it's there. Challenge has nothing to do with it because chances are you'd still look for a way to cheese the content. It's more a matter of the developers implementing trash in a way that you have to face the mobs while reducing the chances of skipping mobs. I guess zone drops would also help, except they made all loot come from treasure chests so that's not going to work.If they really want us to not blow through it so quickly then up the challenge or give trash a reason.