I've actually taken some time to refine the Horde rules. I was never entirely happy with them, but they work as is for the purpose intended. Using the Horde Traits presented in the DW Core Rules and in Mo/tX as a basis for refinement, the Hordes I use have more tactical flexibility than just "big tar pit unit doing big inconvenient Damage". Hordes I use are also threatening down to the last dozen creatures or so, though only to a single Marine.
Conceptually, Hordes make sense, as there are many instances in "fluff', short story, and novel where one can read about hundred, thousands, even hundreds of thousands of troops being thrown at the enemy (or vice versa). With Only War looming on the horizon, it's only a matter of time before someone organizes an all-day war zone, with tactical dispositions, assault and counter-assaults, supply and communications, counter-intel, etc. I know Hordes aren't meant to represent specific numbers, but if one looks at the creatures in a Horde relatively (1 Magnitude equaling 3 Scrawny creatures or 2 Average creatures) then the volume of fire placed downrange (or meted out in melee) to eliminate the threat of a Horde puts a new perspective on Mag 42. With this in mind, Space Marines could be used as Hordes, too, but as they're Hulking it equalizes at 1 Marine per point of Mag…42 Chaos Space Marines wouldn't be used in Horde format though, each one of them would be the near equal of a Deathwatch PC, and getting over on them by doing Mag Damage to them would do them (and the PCs) a disservice. It's bad enough trying to run 9 adversarial Chaos Space Marines, however, so I can't imagine the headache of using 42. Hence the advent of Hordes. Hordes rally around 4 or 5 CSMs, and the PCs have something more to worry about than just Traitors: lesser troops blocking their LoS, climbing all over them, clawing at their legs and generally being a nuisance to their movement and ability to address the threat of the CSMs. I have yet to do it, but…
Grappled opponents are easier to hit. I've been thinking about having CSMs Grapple PCs, giving the Hordes to +20 to hit. Or having the Hordes bog down the PCs (again with Grapple, since 10-12 Average-sized individuals could "peel away" from the Horde specifically for the task) and giving the CSMs the bonus to hit. Raveners, Tau Crisis Suits, anything really, could Grapple a Space Marine, and there's nothing they can do about it until they take control of the Grapple or break free, leaving them exposed, vulnerable, and preventing them from contributing to the demise of Horde/Elite foes. But this is the stuff of Space Marine legends, and if a Space Marine had only tales of "I bested this creature in one-on-one combat, and this creature…" and so on, it would be kinda boring. Being able to relate the epic last stand against the final assault of Oberron Galt and his Horde of Ur-Clatch at the Battle of Moniker Wall is not so boring…at least not to the soldiers of the Imperial Guard. "A Space Marine would go on to say "Oh yeah? Well I…"
The other thing to remember is that the Horde rules are the biggest set of Guidelines in the whole game. They can't possibly meet every GMs' needs, even with the addition of more Horde Traits in Mo/tX, and become rather blah after a while…telling the Players they are up against another "Massive" formation becomes a set of statistics (+30 to hit, and so on). Telling Players that a Horde formation is spilling into the burnt-out shells of the buildings to their right, hugging heavy cover and preparing for an assault that will likely take them from multiple directions, that is capable of dividing their attention between specific targets within the Kill-Team, and seems to be utilizing specialized assault gear (nets for Grappling) means they Players have to think about more than, "I shoot. I get +30 to hit".
If anyone is interested in seeing what I've done to refine Hordes feel free to PM me.