Some people (myself included) register for sites and never post, just to;
[*] have access to searching
[*] setting the amounts of posts-per-page
[*] have access to the pictures/files attached to posts
[*] display the forum without ads
[*] have access to all the registered-user sections
[*] keep a place-holder for the user name
[*] having an earlier "member since" date on the profile
[*] And finally, these users also have an account created so they can follow topics that are of interest to them. If a site has useful content where people are constantly developing a product you use (for example, a ROM for an Android phone), and you have nothing to add to the conversation, but you would like to be notified of what others are saying about it -- because you had also downloaded and installed that item -- then an account where "notification of replies" or "followed threads" options are enabled, can be very useful.
We sometimes visit the forums every day. We sometimes only visit once a month... or every other month.
There are many forums that I visit regularly, in which I have not made a single post at, because of what I've mentioned above.
And, you can be assured that a year later, when you go to purge accounts that have not made a post within 6 months, you will find some people marching up to your door with pitchforks, angry that you deleted their accounts... lol
[*] The spammers that Gizmo mentioned, are called, "Sleepers."
[*] The user accounts that I mentioned, are called "Lurkers."
There are many of both types. You need to decide how you want to deal with both of those two groups, as if they were a single same group. One way is to display a notice during the user creation pages, that they must create at least xx (10?) posts on their account within the first six months, to avoid being auto-purged for non-contribution / inactivity. Another way is to just realize that those accounts exist and just ignore them -- while instead focusing on just the garbage posts that might be created and dealing with those.
Think of all the user profiles on facebook/twitter/google+ where the person never posts.
