Except the daemon can leave a taint on your very soul that lingers and grows, the warp isn't really evil or good, but it is rage and hate and passion and malice and love and everything in between, the warp is not just an extra dimension, it feeds off of ours and daemons are solidified extremes of emotion, a blood letter isn't just an alien, its rage and blood lust and hate made manifest and channeling those powers draws your soul to the attention of the Gods, the most extreme of these manifested emotions.
All warp powers run this risk though, using any psychic power will corrupt you over time unless you're very careful, but directly drawing out a daemon is going to do that quicker.
I'm writing this with my fluff bunny furry ears on, it just goes against the spirit of the universe they have created.As far as other armies go... look, I think GW has been pretty clear that they view this as closer to a competitive roleplaying game than a true, hard-minded, tactical wargame. All that "Forge the Narrative" stuff, the frantic pace, the possibility of army lists that are truly broken and un-fun. Your army isn't meant to be optimized and cut-throat; it's meant to be a character.
Why would Chaos Champions devote their lives to trying to get the attention of Chaos Gods when some random Jonny can just stick out their thumb and flag down a passing Great Unclean One. I can't comment on how it will work in the game because there isn't enough information about it yet, but I do feel sorry for Chaos Daemon players who potentially are going to have all their best toys shared out between everyone else.
Oh and I evidently wasn't quick enough correcting that Black Templar mistake, you know post in haste, repent at leisure.
So don't use it yourself?
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Getting the attention through sacrifice and devotion to the gods, so that you may petition them for power and immortality is one thing, focusing your will (and sacrificing your life and almost certainly your soul) to summon a powerful daemon into your control for a few moments is entirely another.
Ok, fair enough.
However, I've got to disagree with you there. There are plenty of examples in the fluff of shades of grey between good and evil, of characters using the arcane laws of Chaos to push it into their service. Like I wrote earlier, check out Eisenhorn. It's just not that black and white.
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Which is pretty definitive of 40k. After all, there are no good guys.
Eldar? Arrogant, self interested, and happy to commit genocide to save even one Eldar life.
Imperial Guard? Troopers are cheaper than their equipment. Let's expend them in their droves, even if it doesn't win us the battle.
Space Marines? Yeah they're more than a little fascistic.
Tau? Insidious.
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