Blocking out Google would have more impact, because it affects a site nearly every internet user uses every day. Still, blocking Reddit is an excellent demonstration of a site which will directly be affected by SOPA (and the impossibilitiy of policing user-uploaded content for copyright violations) and news outlets and the media would have at least one significant example to point to of what the SOPA opposition is talking about. Better than Reddit do nothing, certainly, even if it's only a PR move.