From Forktong on the first page on this thread: "Battle Magic: Probably gonna ditch it along with defensive. The idea was to have an ability to be a generalist instead of a specialist."
So, I suppose we can ignore these two skills until the next update.

@Janju: I usually don't like skill trees because in way too many cases they force me to put points in skills/talents I just don't want. Also most of the time the "early spells" become obsolete once you unlock the following tiers. Some games do it right imho by giving these early spells utility that late big nukes won't have but unfortunately they're a minority.

I think the DOS system is good because if I want to learn, for instance, "Ice Blast", I have to find the Ice Blast book, then put points in my water sorcery skill to boost the spell. To me it's a clear and efficient system. I don't have to plan something like "ok first I'll buy "magic gills" then "douse fire" so I can learn "ice blast"", which seems a bit silly to me but I suppose it's mostly a matter of personnal tastes. Another result is that skill trees favor specialisation in one specific tree, but I think it's usually more a designer choice for easier balancing.
I suppose one good thing with skill trees though is that they give a clear view of the progression path, and thus good sense of progression in general. On the other hand, that's just visualising the obvious so that's not much of a worthy point imho. Again, it's probably more a matter of personnal taste than anything else.

I don't miss "dark & light" magic, imho it's more a trope of manichean fantasy, to me Healing being a water spell makes perfect sense in a context of magice based of the 4 elements.
I agree that at the moment, which npc sells what seems a bit random and not intuitive, but I think it's more due to the game being in an early stage than the result of poor design.
Also the tooltips reflect in real time the result of your leveling up skills (for instance fireball will do more damage after you put points in fire sorcery, the tooltip will show that before you validate your choice) so it seems transparent enough for me but I agree more clarification wouldn't hurt.

I definitely agree that in the current build, diversity is not encouraged, due to some stats/skills/talents being must-have while others are plain useless. That's probably my biggest concern about the game right now.
I suppose the next update on skills/stats/talents will bring more clarification, and gives us a clearer image of which direction the devs are taking. I think it's necessary for all of us to "reffine" our feedback.