Buddy starts would be used if there were no air supply available (GTC or Palouste), to start any engine. It is standard practice to 'cuff' a prop, before shutting the electrics off with no GPU available for just such an eventuality. No electrics would mean you would be unable to motor the blades to the air start position in the worst case scenario.
I can see no reason why a buddy start could not be used on an engine with a broken starter motor, but am willing to be corrected. It would not be a reason to use a buddy start where the ability to windmill start the engine is available. As previously mentioned there are major issues with the amount of crud thrown up and if you watch the videos the tail of the donor aircraft takes a right pasting!
I have heard that the intake guide vanes being effectively 'closed' would further complicate a J windmill start, anyone care to elaborate?