Think about your audience, what is the primary audience (in modern times) for literary work? Normal people, simple, hardworking grunts. The concept of Protagonist vs Antagonist is one of the most, perhaps the most, basic - basic - basic things that you can have, and need, in a story if you want people to understand what you are doing. Too many characters of "equal" importance is confusing for most people, they want a single Hero to root for, and see him or her prevail over evil.
As for roleplaying on forums (which I don't know very much about), I don't see the problem in having a main character that you control yourself if you are the creator of the roleplay. The same theory applies, people want to see the hero prevail, they will connect with him or her sooner or later and forget that they don't get to be the "star of the show". Sometimes the supporting roles become way more popular than the protagonist, name a person who gave a single F about Frodo in Lord of the Rings. He was the protagonist, and we all rooted for him, but most of the supporting roles were the ones that were popular.