I don't really know. I have been using them as that, and they have been shot a few times, so I suppose that's good. On the other hand they are only T2, so it's very easy to shoot them and take them out with one shot. This way your opponent can force you to start taking rout tests quite easily.
I only have two nurglings in the warband, and I don't think I'll get too many more, I'd probably prefer human cultists if I get more warband members. However, there are instances when having cheap warband members can be a good thing, so I'll probably always keep at least one.