There are, of course, many (including Nader) who will justify his continuing to run for President but he's clearly not amassing a larger following as time passes and those of us who were initially impressed are now recoiling. I spent a good deal of time kicking myself for voting for him, believing if those of us who had, had instead voted for Gore, we wouldn't have been stuck with Bush (who's election was inconceivable to me at the time, hence my luxurious vote for Nader). I've heard enough disturbing information about his secretive and authoritarian tendencies to mistrust him and question his motives.
Even the Green party seems disgusted with him and declined to nominate him after 2000.
His immediately post-election interview was eyebrow raising, as well (his "Uncle Tom" quote), and he strikes me as paranoid and bitter, not traits that impress me in a world leader. Additionally, he accused Obama of "Talking White".