Depends on the adventurer and what the rules are for companion characters in that particular adventurer's game. If it was, say, the Courier from Fallout: New Vegas (where companions can only be knocked unconscious), I'd totally come with them. I probably wouldn't be much use besides moral support and philosophical guidance, but I'd do my best. I guess I could also get a gun and half-heartedly shoot at enemies, but I've never shot anything bigger than a BB gun, so I don't know how well that would go. I'd actually make an effort to avoid the Courier's line of fire, though, so in that regard I know I'd be heads-and-shoulders above all the other companions.