I support Nader because he speaks truth to power and has been a tireless advocate for progressive causes for 30-some odd years.
The point of his campaign is not of 'winning' but of getting progressive ideas heard across the country. Progressive voices are largely shut of the media so whenever he is allowed on it helps widen the spectrum of debate. Nader is also not afraid to attack Bush in ways Democrats wouldn't even consider since they represent many of the same corporate interests that Republicans do. This country needs change and all movements have to start off start of somewhere.
Am I that concerned about Nader swaying the election? No, not really. I trust Nader supporters with their vote. I expect most to vote strategically (i.e. for him in safe states and against Bush in battleground states). Those that don't follow that strategy probably wouldn't have voted anyway or could have just as easily been disenchanted Republicans as they are Democrats. There are also
other safe ways to make a statement. Above all though: progressive voices need to be heard.