Should a user be trusted because he is a quality poster?: I leave this to the community to decide.
No, i think you should leave a neutral feedback if you believe a user is a quality poster and his posts have maybe helped you improve your knowledge, or something similar to that. That is what i read in LoyceV's Beginners guide to correct use of the Trust system
Use Neutral feedback for anything that doesn't mean someone can or can't be trusted. This can be good feedback, for instance when someone helped you out.
Should a spammer be tagged? If a spammer can be tagged, it therefore means a quality poster can be trusted. Is this correct?
No. Spammers posts should be reported, a moderator will delete the posts, if the spammer continues, the moderator knows what to do. I will quote LoyceV again here
Don't leave negative feedback when someone violates the forum rules. Instead, use Report to moderator for rule violations
If a group of scammers upto 50 persons are working together and one among the 50 gets a positive trust. He can in turn trust the remaining 49 scam users and they inturn trust one another. Then, the trust system would be compromised.
It is extremely unlikely that 50 member's will work together on bitcointalk.