A few, related reasons. He's a show of both what's wrong with old media, and what can be accomplished with new media. For example, when the media basically accepted upon just being told that Tea Party protestors shouted racial slurs at the CBC, he was the only one who actually questioned and investigated the incident, and basically proved that it never happened. And some of the ACORN videos may have been false, but it doesn't change the fact that no one else had bothered to do any investigating into the organization. Basically he's one of the first signs of a possible era where thanks to online citizen investigative journalism, nobody is safe from having their authority questioned, and for having their darker side exposed, including/especially the old media, which this would eventually replace.
I frankly think he released the Sharrod video more out of a short temper aimed at NAACP and a big head than out of malice or inentional deception. It would fit his profile, and as I've said before, I find it hard to believe that he'd release the clip knowing that the full video would later discredit it and embarass him.