Isn't being detached from the warp kind of what lost them the War in Heaven in the first place? Why are they striving for more of that?
The Necrons (and C'tan) are complete warp blanks. They cannot so much as touch the warp if they wanted to, which is what also makes them so unique. Since they lack this ability they have become the masters of real-space, learning to bend and manipulate the physical universe to meet their purposes. It is how their ships are still capable of interstellar flight and similar feats that generally require at least some access to the warp.
However, despite the fact that they do not need the warp, it is still something that is beyond their control and thus a threat. The C'tan absolutely hate the Warp. The Old Ones (and later the Eldar) were only able to keep the forces of the C'tan at bay as long as they did during the War in Heaven due to their prowess in harnessing the power of the warp to use against them. Because of this the C’tan began their great work and had the Necrons construct a series of pylons which, if completed, were intended to close off the Warp from the material universe entirely. This plan failed of course, but had it succeeded the immediate effect would would have killed all psykers in the galaxy instantly, sealed off any and all warp entities from being able to influence real-space ever again, and leave all life in the galaxy at the mercy the C'tan.
Back on topic of the OP, the "Necron Pariah" is simply an extension of their hatred for the warp, psykers, and all that. By taking the Pariah gene found in some humans and integrating it into a new generation of Proto-Necron, they were essentially developing soldiers that could be used to directly counter the warp on the field of battle. As it was still in its proto-stage when the Necrons rebelled against the C'tan, there most likely aren't a whole lot of them. It would be cool to see them come back at some point.