Look at the cost involved to get those 24 (or 18 for the reaver) shots:
Warlord, 4x destructor, suecharge: 1050
Reaver, 3x destructor, surcharge: 825
That's a pretty sizable investment especially on a slow moving platform. Assuming you triple on the first turn you get to park yourself in the middle of the board, you fire once if the game goes the standard 3 turns. Unless you play with light terrain there should be a fair amount of ways to block LOS so you might not even be able to fire at an optimal target.
Quote:
Crucially a Plasma Destructor shouldn't be deadlier against most targets than an Imperator's Plasma Annihilator!
That's not true actually. Just because a weapon is bigger and more deadly doesn't mean it is more powerful in all circumstances. A .22 will kill you just as dead as a .50cal but that doesn't make the 50 less powerful. The annihilator is far more powerful with TK since due to the added damage against WE and the removal of RA saves entirely.