In the case of the voidraven I usually took the shatterfields. Same cost as monoscythes and the wound rerolling was forcing more saves, that said my targets are generally all marines, or more recently demons with universal 5+ invul so I wasn't losing out on the ap5. The added str never hurts against vehicles in a pinch, and lets face it sometimes there's nothing else to shoot.
I want to like implosion missiles, I really do. I've tried the full rack along with a list of other absolute nonsense, like caskets, dark gates, hexrifles etc. I figured with low chances of success you just increase the permutations and something rad was bound to happen. I digress. If you see a lot of paladins, thunderwolf cavalry, nobs, mid-sized tyranids (they're out there), tau battlesuits, or 3-wound monsters of any flavor implosion missiles can actually really shine. I wouldn't take a full rack, however. If you're that concerned about these units, I'd recommend one, maybe 2 bombers and put two implosion missiles on each for at least a solid round of shooting out of them. They're just too expensive to bank on any more than that in my opinion. That and void lances/bombs will generally ID or seriously damage the implosion missile's targets anyway so the raven's other weapons and poison from the rest of the army should do the trick.
Looking at the dark eldar "armory" is like looking at a list of middle fingers tailored to certain units. Don't let high points costs discourage you if you see a use for them!