I agree with Robin, it is another avenue to expose content. I maintain a forum mainly due to the fact it is a poker forum and we run private games. I have seen what happens to forums that free chip or in the case of poker, ND bonus and freeroll.
We are not a big forum but we run private buy in games and they are well attended. It is more of a social thing than a money engine. I make tons more off of the blog and website than the forum but, we need a gathering place to share passwords and such. I still try to update with promos and new deals etc. but is usually a duplicate (yet original) from the blog.
It can surely drain you, and if you are cast as a freeroll forum it goes no where. Perhaps if you had casino slot contests and forum competitions it would be more of an asset to you.
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"People who are unable to motivate themselves must be content with mediocrity." ~Andrew Carnegie~