"The client is always right," right? In the consulting world, if we follow this mantra, we are most certainly doing our client a disservice. Your client has hired you for your professional experience, even if it doesn't fall in line with his or her preconceived notions. Clearly, when you're hired to do a particular job, then you must follow-through accordingly. But, if a client suggests a strategy that you believe would steer the project off course, then you've got to speak up.
Being on the client-side in the past, I've experienced both ends of the agree-ability spectrum. When a consultant would concur with me regardless of my stance, it read as inauthentic. And, when a consultant disagreed with me regardless of my stance, it felt unnecessarily pretentious.
I am not suggesting that the best approach is always to meet in the middle or compromise. My belief is that clients are paying consultants for genuine expert advice. An ideal client/consultant relationship is one in which the consultant understands that the client is the expert in his/her particular industry and vice versa. Information should be exchanged to formulate an approach that brings the respective sets of expertise into play.
The resulting collaboration improves the effectiveness of the projects and likely results in repeat business for the consultant. Perhaps your colleagues may even become genuine friends.