You won't incorporate information from outsiders if they think they're going to get ripped off by insiders.
(It's why I wrote about two perspectives - preferring outsiders to insiders only makes sense if the goal is to create a gambling venue.)
For large/complex enough bets, you won't have a single group of people who know how things truly are - you'll have multiple groups of people who each have (or believe they have) a critical piece of the puzzle, possibly by proxy, that makes them think they have a crucial perspective - and those points of view will conflict, giving each such participant a reason to bet. Taken in aggregate, you get a system that pulls in all those perspectives and distills a single signal out of them.