The ONLY way to truly succeed in life is to first fail. אין אדם עומד על דברי תורה אלא אם כן נכשל בה - You can only begin to understand Torah if you first fail to understand. When one succumbs to the yetzer hara he prepares himself to be even greater than a tzaddik bacause we know from Chazal that a Baal Teshuva is greater than a born and bred tzaddik.
The yetzer tov is called "tov" because by following it we become tzaddikim. The yetzer hara is called "tov meod" because by following it and then doing teshuva we become even greater than tzaddikim.
See Igros Uksavim of Maran Baal HaPachad Yitzchak page ריז.
