Many times people mistake BHAGs for 'overpromising', which is not an accurate way to interpret BHAGs.
The intent of setting and communicating BHAGs is to deliver by inspiring others to the same goal, under the assumption that the team is capable of rising to extraordinary performance in order to achieve said goal. The key to such performance appears to be confidence in the players, such as for example, if I say 'let's go to the moon', they respond 'when are we going and how do we address the milestones needed to get there'. In the absence of an extraordinary rise potential, a form of sublimation, the 'overpromising' serves as a filter to identify and recruit only those who are capable of and interested in learning peak performance. The peak performers come forward. This could be a useful part of a leader's 'who hat'. I do not believe that achieving OSE's stated Promise of Open Source Collaboration, namely something that would bluntly speaking be 'saving the world', is possible without building a Level 6 Leadership team of ordinary people rising to extraordinary performance.