Holy trinity exists because it is natural appearance. In real war you have tanks, armored vehicles; you have artillery as damage and planes too; you got hospitals, mediсs to treat soldiers. Skyrim is most freak game if we consider RPG part. Just imagine, if Zenimax aim for immersion, how can it be, that dedicated assassin that spent whole life to master bow arts will become a healer in a moment? Why noone NPC in TES never participate on more than one guild? If you warrior - you warrior, if mage - mage. And only player is GOD, who can all. Probably you the one who missed fireballs in Assassin Creed. I'm not telling even about balance aspect of game: party of 4 templars can switch between dps, tank and heal in a moment, providing instant heal for whole party or damage burst to kill any enemy on sight. Such game won't go far.