Members for members sake isn't really helpful to the site. It is content that drives the site. All you have to do is look at PH right now -- tons of members, no one posting. So nothing to read and be educated or amused or saddened by.
When we started this forum, we honestly thought at the end of the first year we would probably have 50 members. But we thought "Hey, if they are all contributing thoughts, ideas, recipes, etc. it won't matter that it is 50, it will be great content." So to have guests and lurkers might be nice for the guests and lurkers because they have great things to read, product reviews from members who spent some serious time, they see questions answered, photos posted, how to on cooks, etc etc -- but they take only and contribute nothing. I want to reward the contributors.
And yeah, someone could go in and post a lot of hellos, but they never know when a drawing might be and, as a retired attorney, I can probably write a rule that says your posts will be reviewed and if more than 50% are "welcome" posts, you are disqualified (I have never had a problem being a Blue Meanie
and Bent isn't the only decision maker on these issues.)