I support the idea of a Helpful post button because it adds a qualitative dimension to the post counts, titles (Evangelist, Legend), and program affiliations which members may use to assess a post's credibility. Positive reinforcement encourages more of the same, and this tool should increase engagement of members who are high quality, but infrequent, posters.
We already have a triangular Unhelpful button, located in the lower left corner of every post.
I don't think TalkBoard's role is to simply follow the results of a poll, but the number of members who have voted in the poll approaches the number who voted in the most recent TalkBoard election, and it should be given some weight.
Alert a moderator is not the same as unhelpful.
Unless the post is edited there is no public record to offset a helpful indicator. If you're suggesting that moderators be tasked with updating posts for inaccurate or dated info, that it a significant reach beyond what I understand the moderator's role to be. This might not be welcomed by general membership of FT (how long before accusations of censorship arise?) or by moderators (significant extra workload and quite a change in tone of interaction with other members).