Lawrence O'Donnell explained last night why this is a dumbass move. The Senate intel committee in a very rare move allowed the chair and co-chair to subpoenas anyone without the full committee say so.
Cohen was involved in all of Trump's Russia business transactions over the years. He was negotiating a Moscow Trump tower until the beginning of the campaign. He knows all about Trump getting loans from the Russian crime families.
Marshall over at Talking Points Memo also agrees that Cohen is the man to investigate/