foo(std::shared_ptr<bar>) is copy-constructed as part of your function call (bumping the refcount) unless copy elision is both available and allowed. It's only ideal if you almost always pass newly instantiated objects.
Pass by const reference is the sweet spot. If you absolutely must minimize the refcount bumps, overload by const reference and by rvalue.
As for shared_ptrs being very rare, uh, no. We use them by the truckload. To each their own!
