There's two reasons why you shouldn't use a traditional marketing or branding agency to unlock your secret sauce.
The first reason is their business model. They make money being creative: designing web sites, logos, content, setting up email campaigns, and determining your corporate color palette. Unless you are specifically paying them to do client research, the time and resources they spend not being creative eats into their margin. Some research is expected, of course, but most of the time the research objective is more aimed at getting you on-board with the final product than in challenging your thinking. When your marketing is based on what you like, too often you end up marketing to yourself. For the creative business model, this is okay because you're really happy with your new web site. Of course, there's still the issue of getting the results you need and expect.
I have a different business model so my priorities are completely reversed. I don't understand how you can build efficient and high-impact marketing without knowing the real reasons clients select your firm. It's why one client describes my process as starting with Step 0.
The second reason is sequencing for better results. Execution is where the rubber meets the road. A better foundation leads to better execution. With a comprehensive and tactic agnostic marketing strategy in place built around why your best clients select your firm, web sites, content, all that stuff can be designed to a much higher standard. Sequenced this way, the marketing and branding agency actually looks much better because they are focused on what they do best. They have to share the spotlight a bit. I can't speak for anyone else, but to get a wildly successful client, I'm happy to share the spotlight.