Yeah in pvp there isnt much point to immo, doesn't generate any real pressure. I mean I guess you can keep it up to get more emberbits but really its a drop in the bucket. PvE on the other hand its just mandatory, has higher DPET than incin so you keep it up.
In pvp though it does around the damage of a conflag only over 15 sec, not super deadly, since you don't need it for conflag slow or dps buffing your other spells like in the past, It doesnt do much for pvp. FnB immo in bgs however is fairly nice, when used in conjecture with all our other aoe spells it helps generate enough bits to keep the spam up and does some damage along side the FnB conflags and incins, plus RoF
in pvp i used it in the past to help against dispells but nowadays you dispell it all in 1 klick, so nope i don't use the spell that often in pvp
and the fear lasts longer with no spells active so some time to place havoc and try to get that chaosbolt in
in pve it's better then inci, generates embers and with fnb it's nice for bomb dps :P