Unfortunately, your comparison is flawed in multiple ways. You're not comparing equal squads (230 points of TSons vs 212 points of
CSM), and magically removing the sorceror from the Sons buys 2 extra contributing members, not to mention you're fighting your guys against TSons, but you're not comparing the damage output of TSons vs the equivalent amount of
CSM's with plasma, which is the real question.
So, ideal targets for the TSons is a marine squad caught in the open with no cover or invulnerable.
5x Thousand Sons + Aspiring Sorceror is 173 points vs 9x
CSM with 2x Plasma guns + Aspiring Champion (with bolt pistol/
CCW).
Vs Marines in the open, No Cover/Invulnerable
TSons = 1.67 wounds per round at 12 - 24"
CSM = 2.28 wounds per round at 12 - 24"
Even in ideal circumstances for the TSons speciality, they get trounced by >35% based on damage output! In every other engagement I can think of, this only gets worse for the TSons.
OK, so, they can't be used as offensive powerhouses. What about on the defensive side of things?
Lets assume that our same units are caught out in the open by the same Plasma wielding
CSM squad and our hapless
CSM squad doesn't bother to go to ground since that's again, best case scenario for the TSons.
TSons = take 1.33 wounds per round at 12 - 24"
CSM = take 1.89 wounds per round at 12 - 24"
But, TSons only have 6 guys and the
CSM have 10. So, if the same squad just stood there taking weapons fire;
The TSons would have 5.33 dead on round 4 and 6.66 dead on round 5.
The
CSm would have 7.55 dead on round 4 and 9.44 dead on round 5.
Again, even assuming
best possible conditions for the TSons and the
worst possible case for the generic double plasma
CSM, the
CSM are both more durable and more killy when compared to TSons.
I believe the only time you get actual benefit out of the TSons is when you're getting shot out in the middle of nowhere with no cover saves with Battle Cannons/Ion Accelerators/Demolisher Cannons/Baleflamers etc where you can deny a large amount of models their normal armour and cover saves. In those very specific cases, granted the
IA and baleflamer aren't exactly niche, then the TSons can lay claim to trading offense for a defensive advantage. Unfortunately, Thousand Sons are almost exactly as bad as people think they are.
Yes, the Asp. sorceror
could possibly make up some of the lost ground with psychic powers, but now you're stealing dice from your
HQ (which you had to take to make them troops) who is far more efficient given that he's got a spell familiar. Yes, they have fearless so they're not going to run. Yes, the Aspiring sorceror has a force axe. Shut up, yes, they all have axes if they know what's good for them. Thos last points means they can tarpit stuff very well, but look at the cost of the unit and then decide if it's really worth it. Then again, they're slow and purposeful, so they can't run, nor can they fire overwatch and having smaller units means you are far more subject to streaks of dice luck. They have a reasonable amount of stuff that can't be mathhammered out, but don't ever argue that TSons have a niche killing marines, it's just not true.