Can somebody explain if I am missing something? This ability lets you ignore Primitive Armo(u)r. But your weapons already pretty much do that, and so it seems that all it would do is give a slight, slight benefit to grenades and unarmed attacks.

It seems worthly if this is the case. OK it costs just 100 XP, but even so it seems nigh completely useless.

I think you're missing the cost of the ability.

100 XP

What more do you want or expect from 100 XP?

Plus it appears the ability only works with melee attacks. The description specifically mentions (space marine) physiological enhancements (which wouldn't do anything for ranged) and the Improvements only work for melee attacks.

Even for 100 XP it's... nothing.

Assuming we are using pre-errata weapon stats (which these abilities seem designed for).and the standard (only, so far) Tau ally with primitive armor, Kroot, your basic combat knife will reduce the AP to 0 anyway. Your fist will reduce it to 1 AP, since Unarmed Master makes your HTH attacks nonprimitive.,

So what this seems to do is give you +1 damage when punching or grappling a Kroot (or hitting it with a rock ).

In any case... it doesn't seem I was misreading it (other than applying it to tanged attacks).

That's assuming Kroot armor counts as primitive at all, which I'm not sure it does.

Primitive (2) on the Body only. Literally the only thing in Mark of the Xenos with primitive armor.

Ah, okay. Pretty sure it's not primitive in Rogue Trader, though.