I think the bit about Neville's talent in Herbology has served several almost unrecognizable uses. The more obvious one being that Moody gave the book on underwater plants of the mediterranian to Neville, so it would get close to Harry.
However, I think J.K. Rowling has been trying to make Neville come across as a loveable dunce in books 1-4, and we're only beginning to see his transformation. She knew we would be glad he's good at SOMETHING because he's so dismal at everything else, but I think she knew (and was correct in thinking that) we wouldn't take someone who's main talent lies in Herbology as seriously as we would someone who's talent lies in Defense Against the Dark Arts.
I don't think Neville is the chosen one. I don't think Dumbledore had trouble with saying what he was saying because because he was lying (besides, for years and years, he's kept it from Harry without lying, surely he wouldn't sit Harry down, say "Now I'm going to tell you the truth" and then spout out a bunch of lies. No way). I do think Neville needs to start being taken more seriously.