Michael Dolan has a great article at David Allen's Coach's Corner entitled, The Impact of GTD on Groups (I would strongly encourage you to add this feed to your RSS reader if you haven't already). Dolan's point is that when groups collectively implement GTD, "they typically experience benefit in three key areas: Accountability, Focus, and Adaptability."
Oddly enough, I initially adopted GTD for myself for precisely this reason. Having left a church planting endeavor that lacked these elements in the realms of direction, organization and implementation, I saw the team-unifying potential of GTD-- especially for church planters and pastors.
Here is how Dolan sees the benefits of collective implementation paying off for groups: