What you have failed to point out in your admittedly informative post is that S6 will Cause Instant Death on toughness 3, multi-wound models. You're also only talking about vehicles and low toughness infantry.
Destroyers wound SMurfs on 2s and Flyrants on 4s, which, you know, is nice...
Also, when you're pointing out that Destroyers and Immortals are equally survivable you're kind of missing the effect that Destroyer movement has on their durability. With the ability to move 12 inches and and the 36 inch range they're able to stay out of range of weapons and still fire, something that Immortals would be unable to achieve.
It's not really that I failed to point it out but just that it's sort of a moot point about instant killing toughness 3. Unless you really intend to aim your destroyers are swarms? in which case you are totally right Destroyes would pawn those. But mmm I just wouldn't feel right besides the warriors need something to shoot i'd leave it to them.
There ARE other multi wound toughness 3 models out there but not enough to make a case for it's usefulness. As I tried to point out the majority of targets you'll face and shoot at to gauge effectiveness.
More than half of the time these are the sort of targets you'll be shooting immortals/destroyers at. Standard Infantry and Light Vehicles.
Against MEQ i already mentioned that the Immortals are better. Yes Destroyer wound on 2s but Marines save on 3s. Immortals beat out destroyers by causing equal amount of wounds against marines + more rends.
Against Flytyrants the Immortals ARE still better just compare
Tyranid Monstrous Creature EQ
Toughness 6
Sv 3+
Destroyers - 15 Shots - 10 Hits - 5 Wounds - 1.66 Dead (+ 1.6 Possible Rend)
Immortals - 18 Shots - 12 Hits - 4 Wounds - 1.33 Dead (+ 2 Possible Rends) WINNER
yes destroyers can easily stay out of the range of small arms fire this is true but other weapons are likely to target them to equally decimating effect.
With necros you REALLY need to value every wound you have. Immortal units carry more wounds improve PO and WBB odds as well as not losing as much unit effectiveness with every one killed. The hiding in the back strategy is more suited for Heavy Destroyers. Destroyers because Destroyers need to move UP to maximize their range the same thing with Immortals.
IF immortals had Rapid Fire weapons instead. Then yes Destroyers would be superior but this is not the case. Hopefully we get a new codex that makes destroyers cheaper and able to form bigger units. That would be a dream come true eh?