Re: If someone is a proven liar, do they deserve to be given negative trust?
Which is why i've said many times that we need a reputation trust system and a trade trust system. At least if we had 2 different systems we would have more things to complain about as people would misuse 2 systems instead of 1.
You bet! The current trust system already leaves some folks scratching their heads. Can you imagine the chaos if we introduced another layer of complexity with a second one? 
Yes, its already an overly complex system -- I agree that any additions would just create unnecessary complications. Some people continue to fail to grasp the difference between trust ratings and inclusions/exclusions, despite there being plenty of educational resources out there and years worth of banter about how they should be used.
Liar or not, I have been trying to be a little more mindful of who I tag these days. Since we don't have an appropriate trust system for reputation, I think we misuse the current system. What other choice do we have though? Alts of banned people get tagged, someone doesn't like someone else and a tag happens, or a guy cheated a campaign, or 100 other reasons.
Also seems to be the case. The trust system has taken on a special significance here because (from what I understand) it is used by campaign managers to help differentiate applicants. But at the end of the day I'm a firm believer that it should only be used to indicate trade risk and not as a tool to silence critics or promote friends.