[boost] [shared_ptr] calling functions by reference

This might be a unimportant question and may have already been answered many times before, but I was wondering about functions that take shared pointers as arguments. Is there ever a reason I would not want to use a shared_ptr reference as an argument? I can't think of any, but is there any? class XYZ; void foo( shared_ptr< XYZ > &xyz ) { }