Two words: replay value. Imagine crawling through ur favorite dungeon with a buddy! sharing loot! I mean sure the loot system is pointless and the game would probably crash twice as much but there is no excuse to not include multiplayer now-a-days. I mean what fun is a game unless I can tell someone their gear is suxor, or 360noskopezroflstomp some noob with the bound bow. Oh yeah patch in head shots too.
Switching gear or spells would be a pain in the ass. Either it would force both players to pause or you would be defenseless while switching. Also there are a number of slow time abilities that would get really annoying for the other player. Although I admit Legendary multiplayer would be amazing otherwise. A support/healing build would be viable, and perhaps even better than two warriors. A dual casted healing spell with occasional casts of Courage or whatever the higher version is and that warrior in front would be literally invincible while the healer still has magicka.
Take out the slowtime effects and equip your gear before combat. That's what they did in RDR and it worked there. Though I would hate the online multiplayer like RDR. It was a great concept but there are way to many a$$h0le$ in there. It never occurred to them to make it "friendly fire isn't".
Anyone who chooses graphics over story... deserves neither.