I must admit, I am baffled, the general tone here is the EXACT opposite of my personal experience with the skill line. I played a mage to 21, even cleared a dungeon solo, as a light armor mage with no deff skills, and only because of summons. Imho the summons are amazing. The clanfear (i prefer clanfear over unstable familiar) deals about 1/4 of what you do with normal attacks, tail swipes regularly for small but consistent aoe and if you compare that to other classes a 25% dmg boost to auto's is something only very few have. And the winged twillight hits even stronger, + gives whole party 10% extra magicka regen. The costs, while high, are reasonable since you get 15% max magicka back when a minion dies, with the innate 26% magicka cost reduce on mages that’s about half of what they cost. The winged twilight has relatively good AI and Instantaneously moves out of aoe and rarely gets targeted anyway, and the clanfear provides an excellent meat shield if the tank is in trouble (mine lasted through 5 hits from Rorick, buying us almost 10 seonds when the tank decided to die). And besides, what ELSE would you want to take anyway??
You have a) Crystal blast for steady magicka dumping (so far in the beta that was the best scalling and most magicka->dmg efficient spell out there)
Mages Fury for executing
c) Lightning splash for aoe damage (better than daedric curse, morph it to 6 meter diameter and that thing fills half the room, and is really cheap at -36% costs, and profits both from 6% bonus dmg trait and 6% bonus dmg aldmer)
and if you want d) Surge for auto attacking boost. Everything else is dps inefficient anyway.
Ofc lightning form or encase can be useful in some situations, but then you can just drop clanfear.
AND you have 2 slots left for endurance dumpers (or Elemental weakness if you fancy destro staffs... elemental weakness is pretty damn sixy). I, personally, always ran with both clanfear and winged twilight and i visibly did more damage than most people, especially steady damage.