Sometimes the SEO world folds in upon itself and starts doing weird stuff. Such a thing happened just recently when Matt Cutts made an offhand remark about Archive.org. Michael Martinez sums it up pretty well.
If you read his blog post and come to the conclusion that you should NOT block Archive.org because you don’t want to be accused of being a spammer then let’s go back to school. Matt Cutts wasn’t sending a signal that anyone who blocks Archive.org is a spammer. On the other hand, simply blocking Archive.org isn’t going to solve all your problems either.
Clearly, whether you block Archive.org or not is a decision you have to make for yourself. There are legitimate reasons for doing so. Spammers – some spammers – do it, but they do it for a different reason.
When it comes to SEO, don’t follow the crowd. Don’t listen to the myths and turn them into religion. Just do what makes sense.