Many agile methods miss to address the issue of portfolio management. This is not such a surprise as many organizations miss to address the issue all together. Agile thinking pretty much assumes that the only intelligent way is to work on a single project at the time, and that projects should face delivery or cancellation as soon as possible. After all having too many projects in the inventory is one of the main sources of waste. While all this is very much true, the life on planet earth is sometimes somewhat different. The impression "I heard it through the grapewine" is made famous by Marvin Gaye. The impression means information traveling through gossip and rumor. In some organizations this seems to be the primary channel for portfolio management.
Team is defined as group of people with shared objectives and goals (term "team" is misused a lot!). Teams become superior over individuals when they share these goals and thus have a common direction. Agile methods take advantage of this phenomenon by planning for short enough increments and gaining commitment by having estimates given by the people doing the actual work. This is called time-boxing.
When a team in large organization is piloting agile methods it is usually working as a shadow system parallel to institutional system in the organization. In this case the above mentioned "I heard it through the grapewine" -effect can be very harmful. Each team member will have several inputs about the future of the current project, as well as information about the future projects. Along the current increment short notice invitations to future project meetings will arrive and so on. This makes agile planning unmotivating and even barking mad. Of course the same applies to any method of planning under these conditions. However time-boxing would be perfect tool against "I heard it through the grapewine" -effect, allowing organizations to harness the power of small teams with shared goals and direction. DeMarco calls these teams Jelled (DeMarco and Lister, 1999). If new announcements and meeting invitations are only made public in the same natural rhythm as the team already operates, their negative effect to Jell would be minimized. Who is responsible for this rhythm? Top management, middle management, project management, line management, team leadership? I think this needs to be considered independently for each case, but it is a joint responsibility and everyone should understand the reasoning.
"Increment duration depends on how long you can keep the change away". Allowing rumors and gossip make this a very short period.