The reason intelligent players build this list is that the rest of the faction teaches them to.
Grey Knights have three 1 DP detachments, which is a lot, and every piece of coverage about them, including our own, correctly describes them as cheap focused packages you bolt onto a bigger list. That description is accurate. Argent Assault, Fires of Purgation and Immaterial Interdiction each cost a single point and each supercharges one previously neglected unit, and they bolt onto five of the six codex detachments perfectly well.
They do not bolt onto the sixth. And the sixth is the one readers most want to play, because Warpbane Task Force is the flagship: the biggest toolkit in the faction at six stratagems and four enhancements, and the one most often described as the faction's best.
So the trap has three legs:
The lesson is true and generalises wrongly. A reader learns "1 DP detachments bolt onto anything" from five correct examples and applies it to the sixth.
There is no tag to catch it. In a faction with Unique Tags, an illegal pairing is usually flagged by a shared tag printed on both entries, which a careful reader will spot. Here the entries look completely compatible. Nothing is printed anywhere that says these two cannot go together.
The failure is invisible until you total it. A 3 and a 1 do not look like a conflict; they look like a small list and a big one. You have to actually add them up against a budget you may not have memorised.
That combination is why we give it a page rather than a footnote, and why no Grey Knights detachment page on this site suggests a pairing with Warpbane Task Force, in either direction.