Please add the following to your list:
Pro:
1. Builds stickiness and daily visitors. Much easier to gain traction in the marketplace with return traffic then having to constantly bring in new eyeballs.
2. Punters are much more likely to support you and your sponsors if they view you as a friend and fellow community member rather then just a site they stop by for odds lines at.
Cons:
1. An enormous amount of work, supervision and up keep. Far more work then most people envision or realize if you have any real number of participants.
2. Much higher stress levels. You are dealing with a lot of different personalities and not all of them are pleasant. Its a balancing act to maintain a lively enviroment without it becoming a hostile one.