Perhaps more annoyingly, I've started to believe it depletes the mythos a little; it steals the majestic legendary feel of the setting, in favour of some arbitrary ironclad fact that is either conflicted by X, or destined to be retconned by Y.
Heh. That's why I'm not in favour of definite canon.
I understand the idea and can see the draw there, but I can't agree with it. As a history buff and a guy with a memory for details, I am my groups' resident fluff expert. Whenever there are conversations ("arguments" might be too strong a word) about the fluff, or discussions of major events in the 40K timeline, I am invariably the one who settles the disputes -- and its very hard to settle such disputes when there is a dearth of cold, hard facts to rely on.
And to me, there's something much more majestic and legendary when there
are hard facts involved. Here's an example.
"The full host of Horus' Traitor Legions was arrayed against the walls of the Palace, their numbers increased by the presence of Imperial Army brigades sworn to the Warmaster's service. The Word Bearers drove before the army a host of daemons and warp-spawn, and behind them, Titans began their initial bombardment of the walls, seeking to force a breach while simultaneously keeping the heads of the outnumbered defenders down."
Or.
"The full host of the Traitor Legions -- over five hundred thousand strong, the greatest collection of Astartes that the universe had ever seen -- was arrayed against the walls of the Palace. Interspersed amongst their ranks were fifty-three full divisions of the Imperial Army, adding another two million men and thousands of tanks and artillery pieces to Horus' order of battle. The Word Bearers drove, before the army, a host of daemons and warp-spawn, and behind them, the full strength of three full Titan Legions began their initial bombardment of the walls. The defenders, outnumbered more than five-to-one, were forced to keep their heads down while the Titans sought the first breach in the walls."
The first one impresses the reader with the fact that the defenders are outnumbered and Horus' army is nothing to sneer at. The second makes the situation sound truly desperate and thus the victory, when it comes, is all the more epic in its scale. But that's just me.

We are iron. We are the gauntlet cast in challenge. We are the hand that holds up the Imperium, the hand that folds into a fist when its enemies threaten. We cannot be bargained with. We cannot be reasoned with. We don't feel pity, or remorse, or fear. And we absolutely will not stop until you are dead.
Hmmm, I can't decide if you're a genius or a sadist.....or a subtle blend

My WIP Thread: ETL II -- Tactical Squad