I'd like to chime in that, without cheating and googling the area you're about to enter and enemies within, a first-time player (or someone who's understandably forgotten some of the hundreds of hours worth of fetch quests etc. and what enemies lay within) isn't going to know what to plan for in any particular moment. So picking the right type of staff or loading the right abilities onto the action bar isn't really viable in most cases. If difficulty is a concern, I just don't understand why...? There's always been a downside to taking some of every spell tree, in that you then have a bunch of weaker spells from each element and no truly powerful ones from any of them; that felt like plenty of payoff if you chose to do so in order to be prepared for every enemy.
Myself personally, I take the abilities and spells that I find to be entertaining or fit for the character I'm playing. So in my case it's usually "okay but I want to have access to Barrier / Fade Step / the Focus Power... but I also need access to Energy Barrage and my elemental attacks" and that gets super frustrating when I've only got eight slots to use but several powers I want to test out. (It reached a point where I intentionally wasted skill points on passives and/or spells I didn't really want because I saw no point in taking more skills I'd actually wish to use.) Sometimes I just want to go back and use one of the weaker or super-specific-uses-only type powers. In previous games, I could do that easily and it made my character feel more like an actual character that I had a hand in creating.
I should mention it's also annoying for non-mage playthroughs, because you don't even have the magical element to explain why suddenly your archer has forgotten how to deal more damage from above but learned whatever new trick, etc.
I'd say I'd love to see the spellcasting from the radial menu return as an optional gameplay difficulty modifier, but I'm still fairly bitter that they pushed a 4GB patch that didn't even include the new custom difficulty options and instead put it in the paid DLC.