I have a view similar to b$'s, but I would further add that I think the "meta-plot" of the show and the tension between Jacob and MIB that we finally were exposed to in the season finale is the backdrop for showing that the notions of integrity, courage, humanity, bravery, and many other qualities that are often portrayed as noble and uniquely human, are the things that we should really value and strive for as people. At the other extreme is the philosophy that "the ends justify the means" represented most directly by Ben and his Machievellian style of management.
MIB and Jacob apparently have a philosophical disagreement over whether people are good or bad. The others make very strict judgments about which Losties are good or bad. However, many of the Losties, and especially the ones contacted off-island by Jacob, are complex characters who have done a lot of morally ambiguous, if not downright evil things in their past. Yet time and time again on island they end up rising above their fears and pasts and doing some very noble -- Sawyer, Kate, Jack, Charlie, Hurley, Jin, and others have sacrificed their own well-being for others (although they remain conflicted at times and are not all good). Compared to Ben, Widmore, and the others, who repeatedly use people and discard them for some overall "good" that we don't know entirely know about, our favorite characters strive to be at the end of the day good human beings. So I think this is what Jacob is trying to demonstrate about the human condition -- that even people with sordid pasts can have a good "core."
NOt sure if that was 10 sentences or less, but that is what I've gotten since rewatching the series after the s5 finale.