Carnage staggers Atlas, knocks down regular mobs so they can't shoot/close when i'm reloading the grenade launcher.
The carnifex is just for high armor targets/sniping.
The striker is for killing groups, shaking things out of cover. You can biotic charge into the group, then point the striker at the ground and hold the trigger down until they're all dead. Or you can just stay back and kill them from range with the striker. Or you can biotic charge, then heavy melee everything. Or you can sit back and headshot each one with the carnifex.
I guess the real answer to your question is this: BECAUSE IT IS FUN. I imagine a pure melee build with biotic charge would be more effective but I find melee builds boring. I've played this game enough so that having fun is more important than leading the scoreboard(which I usually do with this build anyhow).
On a side note, striker doesn't flake out like a falcon does when you're not host. The flip side is that there is a bug if you go straight from carnage to firing the striker---none of the striker rounds register---which is another reason I use carnage right before reloading the striker.
