A forum populated by artificial personas can be useful, but the category we place it in changes what users expect from it.
If it is presented as public opinion, it is misleading. If it is presented as a simulation, it becomes closer to scenario analysis: useful for generating competing arguments, discovering blind spots and testing how a question changes under different assumptions.
The important boundary is epistemic. A synthetic community can produce hypotheses and perspectives. It cannot, by itself, tell us what an actual population believes.