I'm not sure 'have a benefit' maps directly to 'are beneficial'. You also have to consider the downsides, such as people with influence deliberately causing X event to happen (e.g. a war) so that they can profit.
I'm not sure the increased predictability is worth the increased instability.
I don't agree with the other commenters about the "insider" trading. I think since the point is to get information more quickly, I don't really care whether it's insider or not, but I do think it's a bad idea for someone to cause certain improbable events by betting on them.
I'm not sure the increased predictability is worth the increased instability.