Given that I have worked for most of my career in a ‘fancy brand consulting firm’, my answer may be biased :)
I think there is one big difference that I see between the way agencies approach a problem and the way the consulting firms do. Agencies tend to view the brand from the lens of campaign and communication. This is their strength and it is not something the consulting firms are good at. The brand consultants on the other hand should be looking at the entire picture – all stakeholders, not just customers, the entire brand experience, not just media. Where they often fall short ( a constant rant from my friends in agencies) is that they fail to show how a brand moves from Powerpoint to the real world.
But honestly, I cannot refute the fact that there is a lot of BS (both well packaged and not) floating around. And its coming from both agencies and consulting firms.