I hope you are using Extend spell wands when summoning these dudes, it is a shame to have them disappear after only 1.5 minutes. Also extend metamagic and empower metamagic so you can put them into higher level slots as well. Empower should also summon more of them since number you get is a random number and Empower (at least in PnP) works with increasing all random numbers.
Of course! By the time I get to 20ish, I will have the create undead spell in 4 different spell levels. At 4th (normal), 6th empowered, 7th (maximized), 9th (max+emp). Even at 9th they will be worth casting (always summons 1d4+2 = 6 (max) + 6/2 (emp) = 9 skeletons). They are pretty good for tanking anything, even if it one shots them because of their sheer numbers, they are immune to clouds and cold damage so I can nuke debuff the enemies without care. Not worth using extend on them in late game but it is worth using it on the 7th level create undead spell for grave knighs. Graveknights have constitution though for some reason and they aren't immune to cloudkill, probably game oversight
Pure necro is a blast overall!
I also have a necro wizard but I went more for debuffing with enervate and killing with spells. If I get two extra feats I might pick up some of the summoning feats.
One problem with summoning is that they are not considered allied and you cannot buff them with Haste or other AoE buffs. Also the screen is so full of stuff and stuff popping above their heads you cannot see anything that is going on anymore.
Augmented and superior summoning feats don't work on create undead spells because they are not conjuration. But create undead spells are much better anyway because the summonings stack, unlike conjuration spells.
Also I'd definitely suggest a sorcerer instead. If you are going pure necro, you can't go wrong with spell selection and won't ever need anything else. He is also more flexible in battle - you can spam a ton of enervations/skeletons/etc depending on the course of the battle. Undead bloodline's damage reduction is also great with vampiric shield - you can tank and even heal by any fast hitting mobs like manticores,wyverns,hydras,worgs,smilodons etc.