at the point level it looks like, a phasing out isn't going to happen so loosing necrons for a C'Tan isn't a bad thing. Destroyers are no liability
, i've used them in loads of 2000pts battles and they always gain their points back and are a great tactical asset. if there is an enemy unit that needs killing soon their 36" range weapons mean they have a few truns to weaken them and with 3 S6 shots each you are bound to kill something. I've seen a u nit of 10 terminators fall to a volley of gauss cannon shots from a unit 5 destroyers.