Groups skipping dungeon mobs won't be an issue a month after Forever's launch
Because the only material benefits from dungeons are quest experience and loot, there's no penalty for being carried by a level 60. So starting about a month after launch (and for the rest of Forever's lifetime), all the people who are too impatient to kill the near-zero-experience mobs in a dungeon will just get carried by a level 60. That lets them get all the quest exp, get all the loot, and avoid the hassle of finding a tank and healer.
*Bliz even added "Carry Offered" to the group finder to make it easier for people to get carried through every dungeon!*
And then everyone who wants to clear the dungeon in the normal, slower-paced way can do that without the minmaxers hassling them to skip trash or hurry. So don't even worry about people trying to skip dungeon mobs. It won't be a problem starting a month after launch.
And yes, this is obviously a worse situation than just putting mob kill exp back into dungeons.