I always use it in heroics on Dragon bosses, that way the Dragon will spin around and tail swipe everybody. Everyone says "nice work man that was awesome!", and I get added to 4 new friends lists.
I enjoy having two of my main MP restoring abilities (Blessing of Sanc, Spiritual Attunement) being shut off because the boss is attacking the stupid ghouls instead of me...
There are some points where it's okay to use. Wave 9 before Marwyn or the 4th wall right before the ship in HoR are good spots for them as it helps keep a bunch of that crap under control (Though spinning the stupid Aboms is annoying).
Most of the DKs I'm grouped with through the random dungeon thing either don't use it at all (Good) or use it every time it's available on a boss (Bad). Watching them hit it as the last boss in Forge of Souls does Mirror Soul is particularly amusing...same with ANYTHING that either cleaves, breathes, tail whips or creates "The Bad" that you don't want to stand in or want to move the boss away from...
Does Army trigger Bane on Ymiron?
Oh thats kind of what I do, just sit to the side and wait till the blasted little things are all dead...Easy solution is to stop generating threat like a madman. It's being taunted like mad already. Wait for the ghouls to die while doing a light threat gen.
And as for Bane, I know hunter pets used to trigger it when WoTLK released, I don't know if they ever changed it, but I don't recall seeing anything on it.
as to when to pop army? when the last tank dies.
As both a DK tank and DK dps (depends n target and what not.) I only use it on raids as a Dk dps or as 1 final oh shut button cause things are going bad real fast when tanking.