There's been a lot of chatter in recent weeks about how so-called "digital" agencies are or are not ready to be the lead for a campaign. But I think the question is a little off.
Instead the question should be "Why are clients splitting up campaign work based on tactic?"
Despite the maturing of digital agencies such as Razorfish (for whom I work), R/GA and AKQA, today's clients are still sending digital work to digital agencies and traditional work to traditional agencies. And equally bad is having a third company plan and buy their media (sometimes there's a traditional media agency and a digital one). Why is this bad?
OK, the end-zone is down that way 50 yards! Make sure you talk to each other along the way. Now go! [Download PDF] 
I've seen it time and time again: if you want an integrated marketing campaign, how could you possibly brief all the companies and hope they work together and come back with something good and cohesive? The agencies will pay lip-service and say they're collaborating, but there's only so much collaboration that can happen in reality. Each agency is moving fast and really has no time to talk to the others. Plus there is always unspoken political jockeying for protecting the work each agency does have and trying to steal more business from the others. I strongly believe that this model is inefficient (money and time), makes agency people insane, and creates less-than-stellar campaigns.
What should instead happen is the client needs to brief one agency who will create a singular idea and execute on that idea across different tactics and mediums. Therefore the messaging, art direction and strategy for the campaign are cohesive.
Let the one Agency bring in specialists as needed to serve the idea. [Download PDF]
Agencies should not be labeled "digital," for digital is only a tactic. I'd say the same with "traditional." What clients should ask for is strong strategic work that drives results. Let the agency—regardless of its label—decide on who to sub-contract to if necessary.
When we see clients trust their agency and its vision, we witness great work all around:
Oh wait. There isn't a "digital" agency on that list. But there soon will be.
Further reading:
Please feel free to use the above diagrams which I'm making available through a Creative Commons license.