Steven I agree with all the points you have made, but there is also a lot to be gained. I am not going to attempt to draw a line between those that are at the top of the ladder and those which are awfully close and do comparisons by name dropping, because that leaves it open for a lot of debate. For the sake of argument, lets talk about the artists who have successfully sold their work in the past, have won awards, might be thinking of conducting a workshop or want to fill classes.
1. Forums offer a "stage" for sharing one's work and technique.
By doing so, any artist whose work is of a certain admirable scale, will inevitable gain a following of admirers. Admirers in turn translate to possible workshop participants.
2. It is a place where you can perfect your teaching skills. For those attempting to make the jump from painter to teacher, this is a perfect place to try out and see what works and what doesn't. Ideas can be gained on what issues to include in one's teaching curriculum.
3. A "world-wide" recruitment opportunity
I have organized several workshops for other artists in the past and I can speak from experience that it is not always easy to fill a room. It's not due to a lack of willing participants, but sometimes it is just hard to get the word out. When you hear workshop participants traveling through continents to attend a workshop, you also often hear that they saw the instructor's work on the net, often through forums.
4. New Client
Other artists can sometimes be potential customers. I have seen many an artist, who has purchased a painting from a fellow artist. A sale is a sale, after all.
Personal Fulfillment
1. Sharing your knowledge
I don't know why, but many artists have the urge to share what they have learned with others and a forum gives them a less structured and less time consuming opportunity then a full scale workshop.
2. Exchange Information
Technical discussions, new materials, news, what a fellow artist has done, etc. Unless you are a very active artist who mingles around a lot, it's a place to stay informed.
3. Learn new things
Yes, even the most accomplished painters out there need to be reminded that there are ways they can improve. Nothing is more deathly than a stagnant technique.
4. Meeting others
Even though darts fly at times and feelings get trampled on, forums also offer an opportunity to meet and converse with a lot of nice people who share the same goals. Even the "top brass"
might run into someone they would have otherwise never met.
5. Support
Some artists live in remote areas and have made friends on forums, gotten together outside of cyberspace. All people, no matter what stage in life they are at will have insecurities, feel beaten down and at a loss. It is always nice to find support in someone who actually understands what being an artist means.
In Paris, artists of the same school of thought would get together on a weekly basis, discuss, argue, share and help each other. We on the other hand live in a place, where getting together with a like minded person we can actually do shop talk with lives anywhere from 30 minutes to several hours away. And that's if we are lucky!
I believe that even the most accomplished painter can benefit by being a participant in a forum and I would love to see more participation.