I think many of them are attracted by the idea of paying a one-time cost for permanent file hosting, and I think that is more advantageous for spammers than it is for people trying to transfer their coins. The reason blocks are permanent is *supposed to be* for assurance against counterfeits: users can check if the coins they receive are valid by tracing them back to their blocks-of-origin, and from there to the genesis block, and ensuring everything along the way follows the rules. Apart from giving us assurance against counterfeits, permanently storing every transaction only has negative effects, e.g. it is bad for user privacy, and it is a strain on your computer's resources. So I think spammers have a *different* reason for using bitcoin than most bitcoiners do -- or at least a different reason than I use it. I use bitcoin for assurance against counterfeit money, and I see the permanent data storage as, in all other respects, a downside. But I think spammers use bitcoin for permanent data storage, and see the assurance against counterfeits part as irrelevant.