Few reasons:
- It's good when you're logged in - it lets us know specifically what kind of resources to dedicate to the server. There are basically two metrics we look at -- what the server looks like when it's really busy with everyone and what it looks like during "lockout" (no matter how big our server is we're resigned to not being able to design it to perfectly handle thousands of people hitting us at that exact instant. Obviously we don't want to lock out the most loyal users so this allows us to do so without pissing off people who visit here every day versus just when news breaks
- It adds to the community feel. This place shouldn't be a place where thousands flock to on news days. We don't want to be, but understand why we are. It's ok to piss off drive-by internet visitors with an advertisement if it means we create incentive for really good people who want to be a member of a community to sign in.
- It creates *some* financial influx from a class of users who were, for lack of a better word, parasitical. Traffic from non-logged-ins is about 75% of the volume of all of our traffic, just look at the user list at the bottom of the home page. Those 'guests' are drive-by internet browsers.
- We make a few bucks off of ads in the big scheme of things. This isn't a revenue generator by any stretch.