As to the spam, I think it's easier and quicker to see (and disregard) spam in a forum if it's started as a new thread. This suggestion would merely encourage people to post to existing threads (where it would be even more disruptive).

As to legitimate users posting for the first time on already covered topics, I think the very minor disruption to established FTers (let's face it, if the subject looks like a question covered elsewhere, just don't view the thread) is outweighed by being welcoming to newcomers. Also, I've noticed that often people lurk for a long time because they think they have nothing to add. They'll then discover something new, and their first post if often a new thread of great value. It would be a shame to discourage this.