It's near impossible to not get any ranks in necromancy skill even if you only try for other skills and its rather annoying to have a totally useless skill. At least summoning is good for elemental summons too... If you are playing strictly "good" then you get nothing out of the necromancy skill. I think this can be solved by having necromancy skill apply penalties to enemy undead units; If you don't want it to be overpowered the size of the penalty to enemies can depend on how many undead units you field, maxing out when you control 0 undead units. Alternatively have karma determine if necromancy skill gives penalty to enemy undead or bonus to your undead.