There's definitely going to be necromancy summons, but I don't know if the other skill trees really need their own summons. As Pimpel said, summon overload was kind of a problem in the first game. Pretty sure the summoning tree is going to get some more summon types besides the incarnate and totems.
Giving the other Trees Summons allows a really nice direct synergy with summoner.
How summoning works now is that the Summoning skill directly synergizes with environmental magic, making it quite versatile on how you want to use summoning. While I think there are unknowns/not implemented things still (like do levels in pyro increase my summons fire damage?), summoning makes sense to be separate (from elemental magic).
The summons are immune to the surface they are on (makes it great for that element, you can throw down without destorying own summons), totems spread that elements surface + effects (burning, slow, etc), and incarnates get a spell (like fireball) and do damage with their attacks (fire magic damage). You just have to use them strategically.
With a separate summoning skill, that adapts to the elements you use, you can choose how to incorporate it.
Do you want to be a wizard with a minor summoning focus (distractions, extra damage and surface spread), or do you want to be a summoner (they do bulk of damage and strategy) with your spells assisting them.
It does still seem weird that necromancy is called necromancy and doesn't have an "raise undead" type of spell, and since it's only surface is blood (which is basically just water for a surface) spreading the surface doesn't really help unless you're combining with electric or you / your summons can heal from blood.
Currently I'm also disappointed that Cursed/Blessed surfaces do nothing special for summons. I would expect that I could specifically plan to create that surface and then create a powerful summon based on it - but right now there is no added benefit (and in the case of cursed, summons get damaged from it). I think the Cursed/Blessed is pretty unknown for us still, in how that'll play out in the game, but i'd like to take advantage of those surfaces will all play styles (summoning being huge).