Like Jim, I'm always glad when a buyer brings a property to my attention. Even though I would like to be the one that finds them something they love, I also know that my role is more than just presenting properties. These days, with all the technology that is available, most buyers can find properties on their own. I believe the more important role as a buyer's agent is to advise them through the process, from why and how to make an offer, to close, and even post-close.
With that said, if my client makes me aware of something I've already seen on the MLS or through my own searches and passed up on introducing to them, I explain my reasons for why I thought it was not a good fit and, therefore, did not make them aware of to begin with.
Sometimes the information I provide is enough to change their minds altogether (i.e., info the MLS provided that illuminates certain facts that the general public is not aware of through other search methods); other times, they are still interested. In that case, of course I will honor their wishes and help them view the property, and then just advise in the best way I can to help them make the right choice.