Debate: value of consultancies aligned to a single vendor

There are a number of businesses which simultaneously base themselves around implementing a specific software package or vendor (e.g. SAP) and describing themselves as "management consultancies".

How do these firms reconcile this limited focus with looking out for the client's best interests? In particular what is the "value proposition" where the client's problems are not IT-based or where the software that the firm focusses on is not the best for the job?

This isn't meant to be an attack on such organisations, rather an opportunity for debate. I have a point of view that these firms set themselves up for failure by mismatching their capabilities with their branding and market positioning. However, these businesses clearly attract clients and employees somehow. I would like to understand how that deal works.

P.S. Although this isn't a comment on one specific firm, I was stimulated to ask this question after watching the video of the Lodestone presentation at the consultancy fair. I found it difficult to follow the presenter's reasoning on both details and broad concepts.

For example, I don't agree that SAP has such a monopoly on business IT. I also don't see how the firm can have deliver strategy or even process reengineering projects it claims whilst acting in the client's best interests. If everything has to be designed around the structure that SAP happens to have designed into their products, surely that will result in sub-optimal solutions for the majority of clients whose business models don't fit with SAP's vision of the world.