People making many throwaway accounts end up rate-limiting themselves in certain regards. Make a new account and comment with a link as one of your first few messages? Probably going to get auto-killed (someone can vouch for it, I usually do) because it looks like a typical spam message. Similarly, young accounts posting too many submissions end up getting their submissions auto-killed for the same reason (these can also be vouched for, but people are less likely to notice it to do the vouching).

If it's how they want to interact with the site, though, let them. It doesn't hurt anything, just adds more green (and sometimes grey) to the page.