There have already been some half billion threads on the topic of Necromancer race, and from them I've learned the following ...
You should be an Iksar unless:
- you care about how your toon looks and don't like the Iksar look
- this is an alt that you're only going to take to the 40's or so and then use to farm (Erudite *might* be better in that case)
- this is an alt and you just want to get to the mid levels or so as quickly as possible to see what Necros are like (but then never level further)
- you're on Red (then an Erudite *might* be a good choice, if you plan to keep the character at a lower level .. but Iksar still is probably the better choice)
- you don't like having the same race as everyone else on the server
Some people would even argue you should pick Iksar in all of those cases too, just in case you later change your mind.