It's not commen sense FieserMoep, it's down to your personal taste.
To answer your question:
- Been in a bar fight
- Been in a warzone (Iraq, 1-2nd year of fighting)
- Been in a wrestling match
- Been jumped mercilessly by thugs who robbed me.
- Been blown back by a mortar impact (not close enough to harm me seriously).
A fire fight doesn't envoke the same level of panic as being straggled, but it does qualify as a stressful environment. During a fire fight, panic won't grip you if you are trained for it (as a Militant-Savant would be) until the moment you are shot. Sure, your going to be cautious, your going to get disoriented during a prolonged shoot out and yes, you will panic under heavy suppressive fire (as the rules enforce in this game with pinning).
That said, when one is being strangled, pinned to the ground, and other wise restrained the human mind reacts with a certain level of primal urgency, especially once the mind registers the assailant as being stronger than yourself. That envokes the same level of panic comporable to being suppressed by gunfire.
Did you ever participate in a bar fight? There is not much distracting about being in a sleeper hold or some other nasty submission grip.
And I counter: Have you participated in a bar right and tried to text a message with correct grammer while trying to break free from drunkard hitting you? Go ahead and try that, and let me know how you do. That's the equivilent of a psyker trying to manifest his power (his powers aren't natural at his finger tips, typically - he has the warp trying to consume him to worry about everytime he does some vulgar act against the materium).
You also say:
For me it is just absurd that a grapple action completely negates one the most most powerful kind in the WH40k lore and I wonder why the Witch Hunters have near-unique Null-Rods that are less effective than an army of professional Wrestlers that grapple anything left and right.
The Null-Rod is probably a horrible example for your point of view. It's a device whose intent is to expressely allow you to get close to a psyker without fear of being blasted before you get within a few foot of him. Since Psykers are a precious commidity, it's no wonder these things reinforce the idea that getting a psyker pummelled down is consistent with the setting and the rules. Not to mention Soul-Nets at all here.
Now look at the other spectrum - Torpor, used with ballistic weapons. This is the long-range solution that works effectively well.
Now you say:
Just imagine Ahriman being grappled by some generic MEQ and stripped of all his powers.
Poor example again. Ahriman is the best sorcerer in the known 41st Millennium. I'd have to question why he allowed that MEQ to get past his defenses. And even if he did somehow manage that, i'm fairly certain Ahriman would have his buffs, Touched by Fates, break free from the attempted grapple and ignite the meq with a bolt of change under this system.