Many agencies promote the idea that they will solve their clients' problems and allow them to 'concentrate on their business'. However, this is only true up to a point because the process of employing a new agency always involves an investment from the client in time and money. To get the best from a new marketing initiative, the client must also be prepared to help educate the agency and pass on marketing knowledge that can help guide the work and increase the likelihood of success.
And herein lies the problem with specialist agencies. From the client's point of view, if you want some printed literature - you need to brief a design agency. You need to pass on your logo and any brand guidelines and make sure whatever they produce is in line with your strategy. Then, the client decides they need a new website and so needs to brief a new agency and manage a new set of people to their industry and marketplace again passing on the logo and brand guidelines. 3 months later, the client discovers that the printed materials aren't bringing in as much business as they hoped and so they employ a marketing agency to gain a better understanding of the marketplace and then a creative agency to create some new designs which better tap into the marketing opportunities.
Wouldn't it be easier to just do this once? Brief one agency who full understands your business and you can then truly get on with running your business rather than managing relationships with multiple agencies.