In Marketing very few agencies can offer it all (one reason clients waste their money). In fact even the mega companies that do offer it all, need to do it with many divisions. So there is a natural tendency when you have a receptive client to force their needs into your offerings. Or to make believe you have the solution internally vs say referring them elsewhere.
In my many years in direct B2B sales I have never once lost a customer that I referred to another company for help. Even when I had to send them directly to a competitor. Customers appreciate the honesty and integrity, and if what you do offer is top notch they will stay around a long time.
I was working on a project for a client that while I did what I was tasked to do, and did it very well, it was the wrong task and wrong focus. I was not involved in discussions with the client when the work was being proposed. It is possible the wrong people were in those discussions from both my company and the clients company. I did have email exchanges with the client reinforcing the proposed work to assure them the results were going to be. But the work was put on hold and yesterday after 4+ months was in jeopardy.
So I was asked to be in an emergency call to save this work with the CEO of the client. He sent an email to my partner upset and my partner and I had a long talk about it. Then we called him. After hearing his frustrations, I explained the solution options to him. Those solutions probably mean shifting some of the billings from us to external solutions. But they will achieve his business goals and make him happy.
Happy customers mean they are successful, have strong cash flow and revenues, are growing, and thus will stay with you a long time. Too often I hear or read how a business went to an agency thinking they need their help, the Agency sensing money just rubber stamps and forces the clients needs into one of the solutions they offer (Social Media, Digital Advertising, etc) and then fail the client.
In the advertising industry there is significant turn over with agencies. If you get the trade emails you see big companies putting tens and hundreds of millions of dollars in review all the time. Why? Because the Agency that was taking their money tried forcing the Square Peg in their Round Hole. Don't be that Agency.