(( Hey Hochland Jaegar,
To use a spot of empirical data, I haven't found them to really be useful at all. I've used both melee and missile detachments, at points ranging from 1500 - 2500. They often fail to do anything significant, though I have had smallish (15-sized) blocks of Swordsmen do something.
Even Archers I tend to field as independent unit. That way, I can deploy them as needed, instead of being forced to deploy them with their parent company. And, they still can LOS block, get targets of opportunity, and flank charge (if the situation warrants it). Curiously, I've found that crossbow / handgunner detachments do worse then their independent counterparts, as they often will fall victim to LoS problems. Not to mention that there are enough spells out there that if a Detachment / Parent suffers a negative-Trait effect (like Stupidity, Frenzy, or similar), so does the other... many players seem to notice that Parents offer LD / Positive-Trait effects, but forget that negative-Trait effects are also passed along.
More-often-than-not, though, Detachments just fail to assist their parent unit against a charge. And vice-versa. Since the detachments cannot auto-flank-charge now, neither can the parent units. (Not to mention that Parent units cannot declare a Counter Charge to support its detachment.) Worse, Detachments cannot get musicians, meaning they use up valuable movement wheeling / reforming, instead of trying for the Swift Reform.
Anyway, I've found that two, separate units of troops will support each other better, than one unit with a detachment.
Just some quick thoughts I've seen personally. ))
