Responses:
4+ invulnerable is entirely useless occasionally. You got that right. Sometimes its priceless. I'm sure you've had units pie plated or hit by a plasma drop squad, or fired upon by an assault cannon.
When did I claim Ksons are "immune" to plasma weapons? "" means you are quoting. Who are you quoting from?
As for the fighting thing, let's do a little simulation, testing whether or not Ksons can handle enemy elite assaulters. We'll use Raptors as the easiest way to test, since against slow assaulters the Ksons can back up and fire and it isn't a test.
Typical Ksons unit is 9, 8 and a sorcerer with warptime. We'll say its an even 250 points, so 10 raptors will be our foe, give em an Asp champ with a power weapon and they have 2 plasma pistols. 2 reasonable units, right?
Raptors jump up to within 12 (if only within 13-18 Ksons will just move backwards and shoot them), shoot pistols at ksons. 8 shots, 5 hits, 3 wounds, 1 unsaved wound. 2 plasma shots, 1 hit, 1 wound, another unsaved wound (let's say). Ksons move up, rapid fire. 12 shots, 8 hits, 4 wounds. Raptors pass morale. Ksons charge into melee (stable platform so they can do so). Raptors and Ksons swing simo. 5 Raptors swing 10 times, 6 Ksons swing 12 times, raptors deal 5 hits, 2.5 wounds, .8 unsaved wounds, Ksons deal 5 hits, 3 wounds, 1 unsaved wound. Raptor boss swings 3 times, 1.5 hits, .75 wounds, .38 unsaved wounds. Sorcerer swings 4 times, hits 3, wounds 2.25. Raptors lose and next round they'll be ground up.
Play around with the simulation. Spend the Raptors points on the Icon of Slaanesh or Khorne, change the power weapon out for a power fist. Make sure to keep the points the same and increase the squad size, or decrease it. As long as the Ksons get the round of rapid fire and the charge it'll go in the Ksons favor.
Not convinced? Fair enough. Let's use a better assault squad. How about some genestealers. They've got enchanced carapace (not useful against AP 3, but to be honest have you ever seen stealers without this one?) So that's 12 stealers to get ~240 points. They move up within 12" to keep the Ksons from backing up and ventilating them. Ksons move up and rapid fire. 16 shots, 10 hits, 5 dead. Sorcerer shoots down another one with his warp-timed pistol, so 6 stealers left. Ksons charge. Stealers strike first. 12 swings gives 2 rends and 6 hits. 3 of the hits are wounds. 1 of the rends is unsaved, as is one of the normal armor saves. This leaves 6 charging Ksons and the sorcerer. Ksons swing 12 times, 6 hits, 3 wounds, 1.5 kills. Sorcerer swings 4 times, hits 3, wounds 2.25. Stealers have lost and next round they will get ground up.
Edit: Miscounted Ksons points, add one stealer and one raptor, outcome doesn't change significantly.
That ought to be fairly persuasive. Ksons can handle themselves against elite assault units. Key to their effectiveness is that the enemy has to end within 12" to ensure that if the Ksons back up, and this lets the Ksons get the charge off. This coupled with their ability to neuter the enemies boss man makes for a surprisingly tough unit. (the Ksons have a few weaknesses, sometimes they can't make the charge move even if the enemy is in range, due to SAP, and sometimes their boss can't make his psychic tests)
Main point:
Chaos has 2 troop choices able to move and shoot 24". Noise Marines and Ksons. Noise Marines cost a little more per man. Ksons have to drag along a sorcerer. Noise marines are less resilient unless they are in 4+ cover, in which case they lose the "move and shoot" part of the role. Its a pretty even contest.
I'm a Necron player, just like you. I'm stunned you can't see the advantages of moving and shooting 24". Have you ever used Immortals? Warriors? Its like day and night. If you've never tried them go and proxy a unit, stat. You'll never want to go back to the worthless rapid fire brick. They get Immortals-lite as a Troop Choice. Their fighting ability is a plus. Their sorcery is a plus. Even their invulnerable save is a plus. Troops that move and shoot 24, and AP space marines, is the main dish.