> (If they passed by reference, you could change to which value a caller's variable was bound, like you can in C++)

I might be misunderstanding you, but I don't think you can do this in C++. References can't be changed to point to a different object after initialization. If you have code like:

  void MyFunc(Foo& ref_param) {
    Foo new_foo;
    ref_param = new_foo;
  }

The assignment above isn't "changing the value to which a caller's variable is bound". Instead, it's running the '=' operator on the Foo object to copy the state from new_foo to ref_param. To demonstrate this, you could run the following to see that the addresses are the same:

  Foo original_object;
  Foo& object_ref = original_object;
  MyFunc(object_ref);
  // object_ref still points to the same address
  assert(&original_object == &object_ref);

Under the hood, C++ reference params are basically syntactic sugar for passing by pointer-value, so this behavior isn't surprising.