What do you do when your client (or prospect) wants one thing but you know that something else is better for them? As an agency we are often faced with this situation, and it leaves us in a quandary. Do we accept money for effort we know will be ineffective or even damaging? Or do we remain firmly opposed? It's a tough situation. Ultimately, the customer is the boss and their wishes must be honored, but an agency has an almost fiduciary responsibility to make sure that the customer knows its measured opinion (and repeatedly if need be). Likewise, an agency would be remiss if it did not offer an unpopular recommendation to a prospect.
We're preparing a pitch right now for a (hopefully) new client, and we're facing this exact situation. We're going to include our unpopular but correct recommendation. In the long run we'll be happy we did.