What harm is done by bringing on an astrologer? At worst it fail to amuse.
But it's obvious you're not talking about the diavlog's impact on you... you're concerned with the poor, unwashed masses who might actually be left to form their own opinions from the available information. Well, that's very nice of you, but I don't think it's unreasonable to believe that it's safe to expose people to views which might be labeled as "crackpottery" by some.
High-status serious people want to associate with high-status others. Allowing crackpots on the same venue dramatically decreases its attractiveness for quality participants.
You can't really give an elaborate justification for why in this particular case it's OK, because signals are shallow.
I declare this Open Thread open for discussion of Less Wrong topics that have not appeared in recent posts.