You will always have cookiecutter talentbuilds where one talent will do generally better than the others, nevermind the fact that half of the talents are (going to be) passive. So - you choose 1 talent in each row, probably gain 2 spells you have to use on cd and be done with it. Now, if you got all the talents as spells, you'd convert into a pretty big priority system and you would suddenly have to re-evaluate priorities as soon as any of the circumstances changes (which can happen multiple times during a boss fight aswell). You could still do potentially alright by just using general spells, but adding the depth would make the skillcap way way higher, which also plays a role in how engaging a game can potentially be. I'm far from being a very good player but I'd much rather have average players do average dps (sufficient enough to do most of the things in game), and leave the cutting edge players (in terms of skill) the potential to do dps which could be completely on another level.