They annoy people, and they have no legitimate reason to be on LiveJournal; their presence is actively against the Terms of Service.
They're using LJ server resources for their own benefit to drive up search engine ratings, at LiveJournal's cost, taking away from the processor time and storage space that would be available to legitimate users, rather than using a domain and hosting they paid for.
They attempt to hoodwink LJ users into giving them money in some cases.
Some of them attempt to lure LJ users into visiting malware-ridden sites and converting LJ users into unwitting zombies in their botnets.
They're taking up namespace; most of the usernames are garble but not all of them.
They're taking up people's time, granted only a few seconds at a time, but these little things add up.
They're forcing LJ support to handle people complaining about them.
They're diverting developers' effort from things that are more useful to the userbase.
They're taking Abuse time and energy to research and suspend, which takes away from Abuse time and energy spent on other issues.
They make it damned hard to find any real human on http://www.livejournal.com/stats/latest.bml
They need to die with fire.