Yes, but what about ranking? For instance, try to imagine that someone got's an underserved redtrust as a payback from a personal issue with someone, for instance. If the trust is showed by default, probably this person will be unable to rank-up, or it can be more difficult for the hypothetic liable. Hey, you said that this wouldn't influence you, and that's awesome, but, being honest, I would think twice before meriting someone with red trust. I guess that is why this isn't shown in all boards because it can subconsciously influence the people about the one writing.
Of course, I do think that the trust system may be shown in all the boards in which transactions are having a place like the very one pointed up by the OP, or any other with money or work or any kind of transaction implicit. Yet, in those ones dedicated to the public opinion, I just don' t see that the trust system have anything to do with that. Please, correct me if I'm wrong, for I'm just a newbie, but, as far as I have been able to understand, the Trust System is a decentralized tool designed as a way of advice others regarding any kind of transaction, so this is a dedicated tool and not one meant for general purposes.