[Nogdar]

Darkness is mana intensive so it really comes down to 4 things to decide whether you use it or not in addition to the DC/SV rotation:

1. If the mob flees: then you just have to - especially if you're the only group snarer. Not much use casting darkness on a non-fleeing, read mostly undeads, mob however.
2. Do you have clarity 2 and/or bard regen
3. Do you have BE gauntlets (you should) and the willpower to keep switching if you have better ones
4. Is the group chaining pulls hyper fast or is the puller slacking/are the dps ripping through mobs so fast you don't even get the time to cast it (since you have to cast a few other spells first, and clicky (BE gaunts) is a long cast for example).

Note that points 2 and 3 imply you're higher level. When you're lower level, and especially before lvl 39 when you get SV, darkness is your 2nd aggro tool that you should always be casting between 2 disease clouds, but SV takes on this job at 39+, it's really an uber spell. Besides, at lower levels darkness spells aren't so hard on mana as Dooming and Cascading are, especially if your group is killing/pulling fast.

It's great aggro, but according to many factors you might not be casting it on every single mob pulled at higher levels.