Mal has indicated that it's preferred it's not necessary for his HE-500, but other planars like the HE-400 and MadDogs benefited. It is apparently beneficial for most some planars (to help reduce the noise floor) and to allow more sensitive dynamic headphones to be powered safely. I can say it works well with my HE-400.
Edited: Sorry for the hasty inaccurate response!
Edited by modulor - 3/25/13 at 2:23pm