First . You don't have to live together to work together!So any talk of media planning going back within " brand" agencies is a nonstarter.( who's table are we talking about?)
In the emerging media market , with multiplatform campaigns and integrated media partner packages it is more important that media strategy, planning and implementation is together.
That being said; any great advertising campaign starts with a razor sharp insight and a great idea ( which can come from anywhere , but most often germinates in strategic planning and creative) and then naturally ignites and cascades cross channel/platform.It is the connection strategy and media amplification that is only great if and when a collaboration occurs between creative team and media/channel planners.
The solution to ensuring creative-media collaboration is NOT for brand agencies to hire a token media person inside .This will only give creative a reason to opt out of any media relationship ; " I already spoke to the media person". The orphaned channel/media person will have no access to tools, no sounding board for media ideas, no natural relationship with media sellers and no finger on the pulse of the rapidly changing media landscape.
Leaders within both creative departments and media companies must ensure the dialogue takes place, and that media educates creative on emerging digital opportunities and that creatives respect and recognize the oxygen that a great media person provides a great idea.