Hell yes! Though I think it should be limited to certain classes as well as certain level/ranks depending on what is being summoned. Also I don't think they should act as the demons do in WoW where they are essentially a pet. I think they should only be able to be summoned for a short period of time with a CD once they are pulled back into the warp but should be able to cause some serious damage while active. Also, I think they should persist even if their summoner dies until they run out of time or are killed themselves.
I suppose, there must be some restrictions in summon, something like - there is no direct ability to summon daemon it's just a CHANCE to summon. We have seen it in DOW2 (Shrines of Khorne).
Anyone play City of Villains when it was running? I could see daemons being handled like the Mastermind's minions. Start with a small, weak one, and as you rise in power, you get more and then more powerful daemons bound to your will.
Oh to walk upon the blood-stained ground.... I believe Daemons should only be available during particularly large conflicts, where the veil between Realspace and the Warp is stretched thin by the constant bloodshed, use of psychic powers, and other such means. NPC lesser daemons would provide reinforcements, of strength similar to the standard enemy NPC's in the area. Perhaps more powerful daemons would be player controlled, of strength similar to the various tanks that may be in the game. Not invulnerable, but one hell of a problem is not dealt with.
Just any character shouldn't be able to. Sorcerers should have the ability, but there could be a risk/reward scenario such as: Level 50 Sorcerer summoning Time/Risk level 40 daemon - indefinitely and without risk level 45 daemon - for a long period and with very little risk level 50 daemon - for a long period with little risk level 55 daemon - short period medium risk level 60+ daemon - have a special ability on a long re-use timer that requires "sacrifices" (player kills) to use, perhaps 5 kills gets a L60 daemon, 10 kills = L65, etc. (up to a point) What I call "risk" could be any number of things, from the daemon losing it's grip on the materium and despawning, to the daemon turning on friendly units. This would add a bit of the unpredictability of daemons that we see in codex books and lore books, but only if the daemon isn't weak enough to be subjugated completely (daemons below the Sorcerer's own level).
It would certainly be one way of dealing with the numbers the loyalist will put against us. In terms of players, we'll most likely be out number 3-4 Space Marines for every Chaos Marine. My Kind of Odds, but with Daemons we can definitely put them Loyalist boys down.
I guess that while in battle or in a PvP that the Chaos faction can have a sub-mision to kill a certain number of enemies or capture a shrine to summon as a helpful NPC, or a Pet by your side, but summoning deamons amidst the battlefield would be an heck of a good feeling, plus it would look awesome.