Do you believe that there will be a need to authenticate transactions before you receive them. For example if someone deposits a mosaic which has a levy based on another mosaic that you do not own/ may never be able to obtain; there’s no way for you to get rid of that mosaic correct? To my understanding that’s how malicious accounts are tagged. But anyone can impose these “branding” tactics to any account (mainly concerned at trolls).
Anyone can send mosaics to accounts but it does cost the usual tx fee so anyone doing that needs an incentive to do so.
Further, just because there is something sitting in your account it doesn’t mean anyone is going to act on it. I could issue your account a mosaic “dumbass”, that doesn’t mean anyone would believe you’re a dumbass just because of that 
But it’s like trash right? Optimally you would want a way to discard them wouldn’t you. Like receiving promotional material in the mail. It obviously cost someone money to publish but that doesn’t mean I want it.