They could work as an NPC race.
I doubt many people who aren't familiar with 40k would find a silent gray automaton very interesting.

Taken straight from the GW website:
"Unlike higher ranking Necrons, the basic warrior is little more than an automaton slaved to the will of their commander. What Necron Warriors lack in intuition they more than make up for in determination and durability. Once orders have been received, they are completely single-minded and will follow commands through to conclusion without question."
http://www.games-workshop.com/gws/catalog/productDetail.jsp?prodId=prod1380042a

Sounds to me like they're at the same level of sentience as Tyranids. Their minds are all in control of something above them and only when left to their own means can they act independently.

No swapping out! It could always be Necrons on one continent/planet and Tyranids on the other but i dont want them completely taken away for any period of time. Maybe thats not what you meant but from other posts ive seen from you it sounds like you want only one available in game at a time.
You could always have Tyranids fighting above ground and underground landscapes that the Necrons have tombs in. Like in DOW1, the Necron HQ was underground they had tunnels they would come out of.

BEST IDEA. Necron monolith awakening random world even

necrons are probably my second most favorite race next to chaos then orks would take third

This is a picture I drew of a necron warrior id probably create.

I am hoping they are more of an NPC because they lose sentience after some many deaths. So the base warriors are just zombies but higher soldiers are sentient.

wow, my topic has so many comments xD, i can't even read them all c: come on gentlemen, keep the necrons alive ^-^

i dont really think necrons or nids should only be npcs. if you can put yourself mentally into the skin of an eldar or marine, then you should easily be able to play a halfway sentinent automaton. in many aliens vs predator you could play the aliens (which arent that much apart from nids) and it was a lot of fun to play them.

the argument is pretty weak imho.

The difference is a Necron Warrior is issued a command and will follow it without question, and do everything it can think of to complete it. Where as the Tyranids all share the same brain (ie the hive mind), without the link to the hive mind the lower tyranids act as wild animals.
Solo Necron Warriors will still fight with tactics without a leader, Gants can't.

As far as I know, if the Necron lord dies, so to do the automatons.