At some point though, it becomes argueable whether or not you should spend the GCD on RoF, in an AoE situation. For example, on garrosh ( hc for me ), I don't even bother using RoF on the 1st set after placing it down just before they reach the boss, for the ember regen. After that though, you swim in embers, and it's better to just tunnel incin ( or Shadowburn havoc into the boss
).
OT:Yes, keeping up immolate is worth it. If you're dealing with quite a few targets ( I.E. shamans on pull ), I believe it's better to just use FnB>Immolate, because you'll be spending so much time casting immolate on all targets, and later on refreshing it once again. All that time is you could've used actually damaging the targets ( I just tend to aoe like a mad man on the pull, untill I can use havoc to Shadowburn the boss ). In the situation of Paragons, it would still be an single target increase to keep up Immolate on everything, which shouldn't be such a hassle as there are usually 3 targets up. When the parasites spawn ( I think people only do this on heroic though ), just use FnB>Immolate on them when they are grouped up. If your group uses a mass grip on them for CC purpose like stuns, this is a very good opportunity to do this. Aside of that, try and use RoF around 2-3 of the bosses whenever possible. All of this is because both Immolate and RoF give essentially free embers, allowing you to push out more Chaos Bolts.