Private variables make extending the API calls difficult. They also make duck punching impossible without at least recognizing and propagating the hidden vars, in cases like $.data() you have to actually understand the meaning of the pvt variable or you'll do the wrong thing. These can be replaced with a common private method that the two public methods ($.data() and $._data()) can call.

Examples:

  data: function( elem, name, data, pvt /* Internal Use Only */ ) {
  removeData: function( elem, name, pvt /* Internal Use Only */ )
  attr: function( elem, name, value, pass ) {