When you add someone young to your staff, their primary business goal is to add variety to their experience. That's partly because they believe that doing so will make their next job search easier, but it's also a bit self-serving: more variety means more interesting things to do. Not really because they are forced to do research and become an "expert" overnight on a given subject, but just because there seems to be a genetic A.D.D. affliction among the creative set.
How does this truth relate to doing great work for clients? Publicly, no one in the right mind would admit this, but expertise usually takes a back seat to the endless pursuit of variety. This is especially true of poorly positioned firms, where each project is different and thus they seldom can learn from doing many previous projects of a similar nature.
At the heart of intelligence is pattern matching. Finding those patterns is possible only if you have enough similar projects so that the patterns emerge like an inner tube under water. Even if you aren't looking for them, they will appear in a way that will be nearly impossible to ignore.
What's my point? Well, it's a sad one, but here it is: I think that many people in the marketing field value variety rather than expertise, and I think that's unethical. And it certainly isn't in the best interest of your clients.