Sounds rather heavy in terms of placeables and server performance issues....
Given that on that server, players had a number of summoning points, with which they could summon multiple creatures, or animate multiple undead.
That was my basic zombie army. I sometimes had more, I sometimes had more powerful creatures. These things persisted across resting, the only way to get rid of them was for me to log off, unsummon them, for them to be destroyed or for there to be a reset. People could also summon multiple creatures with the Summon Monster line of spells, though they had a duration of only a few rounds per level (a character built for it could possibly get a duration of 9.6 rounds per level, while most just had 3, at most).
Lag was not an issue there. I imagine having only -one- summon per necromancer here would not be an issue, either.
You'll note some of the zombies were "Corrupted." This is because I went in and experimented on them, essentially giving them a number of permanent buffs (which can be risky business, as even if I succeed on the experiment, I take damage, and if I fail both I and the zombie takes damage). These levels of variety for undead really made necromancy shine on that server, it's just a pity basic NWN didn't have a similar system. Anyway, in the end, the more variety (not necessarily power, but the opportunity for it is good) you give to necromancy and magic in general, the more awesome it becomes.