There's nothing stopping you from placing extra points into Dim Vision AND Mages. Think about it:
7 important curses: Amp, Decrep, DV, Life Tap, and their prereqs Weaken, Terror and IM. Add in Confuse and/or Attract if you ever use them (I don't) - that's 9 points to start with.
21 points in the P&B tree - 1 Teeth, 20 CE. That's all you really need.
66 points go to the Summon tree. 20 in each Skellys, Mastery and Mages. 1 in Revive, Clay, Blood and Iron Golems, Summon Resist and Golem Mastery. No more, no less.
With all quests done that can be accomplished as early as level 83. If you plan to level to 90 you can have 8 in Dim Vision by then, which will be higher with +skills.
However, my personal preference lies in Golem Mastery for my Summoner's final skill points. I usually get by fine with careful playing and off-screen Dim Vision casting in troublesome areas, and I like the idea that I can make a chunky Iron Golem down the road if I find something worth sacrificing. Otherwise, it makes for an immortal Clay Golem.