Should you invest in Attack for a Necromancer? Personally, I don't. Attack isn't their primary skill, so Defense is sufficient; it's better to invest in Summoning.

You're wrong to do that; it's a big mistake. The foundation for a Necromancer in a long game is: Darkness with Mind Lord, Attack up to Mass Haste, Defense up to Readiness (if facing a Warrior, but Endurance is essential), Education with Attraction. Then, there are several options:
1. Summoning up to Expert. This is a classic, well-known option (no need for further explanation), but it's not always the strongest.
2. Logistics up to Keen Mind. You still need to catch the enemy, and it hinders hit-and-run tactics and the first cast. Against Orcs, Knights, and Stronghold creatures, it can immediately decide the outcome of the battle.
3. Sorcery. Largely due to Lucretia, but it also helps with high-level Darkness, and most importantly, you can use Counterspell to exchange your hero for an enemy hero if the latter is a mage, as the Necromancer already has combat skills.
4. Luck up to Soldier's Luck. This option isn't very reliable due to the 2% chance of it triggering, but it's very interesting. Alternate skeletons under Vampirism (Bloodlust and Endurance, Claw and a +2 HP ring = 3-5 damage, 9 HP) and alternate vampires with the aforementioned Readiness (Mass Haste!) work wonders. You can safely assume that they will stun and put to sleep every other time, or even more often. Moreover, vampires put to sleep with their counterattack.