the advantage of a roadmap is that allows people that can agree on principle on a specific kind of approach can work together as a team towards a common goal under the same project.
People who cant find someone else to agree with them, or dont like the existing solutions can always follow their own path and create their own code. On other hand those kind of people can also benefit from a roadmap because a roadmap is always an attractive force for contributors.
When I (used here generally not just to refer to me) know what your intention is with this code in the future , I will be far more willing to help you out if our goals are common. If not then roadmap can still be useful if I dont want our two projects , mine and yours to overlap.
So a roadmap is a win win situation.