Around level 15 and Summoning is, as noted above, a joke. Necromancy is absurd. For 3 Action Points, you can summon a Bone Widow. With ONE point in Necromancy, it has more hit points (1800ish - Compared to the Incarnate at 17 Summoning having around 1100) and does the same damage (Physical/Poison), the only issue being that it lasts 5 turns, instead of 9.
Even the 'special' summons you find, like the Condor, do not seem to scale at all with Summoning. Combined with the relative low damage of it's offensive spells, all that it has going for it are the totems - Which all die to two AOE spells or one melee round from any enemy.
Three of my characters have Summon Bone Widow now... and it is making Tactician mode simple. Probably going to swap my Summoner to anything-but-Summoning, put one point in Necromancy, and ride the creepy, giant bone spider train to victory.