Not meaning to sound contrive but I don't see how a dwarf entering a "battle rage" would be of much assistance.
I mean, for instance, my marksdwarf is firing shots towards orcs, why would I want him to suddenly stop and engage them in melee?
Why would I want my wounded champion to stop getting away to safety and watch him fling himself blindly at the crowd of enemies he will surely die too?
Well, you might say, it will only work for close combat, well in which case it is redundant still. If we think about this logically dwarves will already be; fighting furiously, fearless and pumped up on adrenaline when they enter battle. Martial trances make sense because, as said previously they reflect a moment of clarity amidst the heat of battle. Getting angry and ignoring wounds is basically what people do in battle anyway.
However, I'm merely playing devil's advocate here, the idea of possession seems likely a barrel of fun and then some. A "damage boost" could really make for some truly spectaculor fights/sieges/kobold theif encounters
Basically, I like it so long as its rare enough not to be end up as some cheap warhammer40k "blood for the blood god" message that flashes up everytime I fight of an ambush.