On avg you're gonna get almost an extra crit per round on 2 groups (assuming 3 per group) cursed with immolation 10 for every 6 single target attacks used by others on the same mobs. Could be very substantial on large AoE and 1ap attacks. Almost like giving everyone flashing blades or something... (edit the crits sans the acc of flashing blades, cause with that acc would really be crazy, besides that'd encroach on CA's job of acc improvements.)
Reason I'll be debating on investing in BB after maxing both CA and Immo. With 16% Crit bonus Kincaid and Selen will already be getting up around 50% with talents+whatever gears might come up. The extra crit% and fire damage from BB at that point might not be as useful as say levels of Pureflame Shield helping party defense especially since Kjartan starts out with 1 level of PS meaning it will top out one level higher than BB will and keep his talents to a tighter focus. Alternatively some ranks of Firebolt also looks attractive for him to use for helping finish off targets that have already been fully cursed and help hold down his SP expenditure. A lot of it will depend on gear and how things look at the point where he's maxed both CA and Immo. Note; Fyona will be investing in Righteous Ferver so stacking on Armor could prove very effective.
Guns! We need BIG Guns! Then again what do I know I'm crazy!