I disagree. Enemy (and friendly) resistances weren't useless in previous games. Moreover, "Rage demons are immune to fire, better use ice" is more tactical, even if simple, than simply being screwed when you have a fire staff and fire spells, which is what can happen in Inquisition. Especially early in the game, since if you want the spells and passives of a certain tree, you'll have to spend a lot of points and levels in just one tree to get them.
Having full Fire Immunity on all Mabari as in DA2 Nightmare is why I never chose to use it. It also made no sense to me after all the trouble one went to in order to obtain Fire Resist Kaddis in DAO, and now all of 'em are immune. There is nothing tactical or immersive about that, IMO.
However, finding creatures that do have Fire Resistance when you are a Fire Mage is going to occur. One may progress, or retreat to reconfigure the Quickbar if other spells are available, If none are available like in the starting levels, then setting op combos are helpful, and Fire magic still will do damage, though at a lesser rate. And this is likely why a Mage starts with a Fire and Lightning spell.