Daryl is a sinner being redeemed--Shane was a one time good guy on an irredeemable path. Once you try to murder your best friend to have his wife, you're pretty much in a corner as a character. Daryl is one of the hearts of the show, whether by design or not. Shane and Lori were not. Also, you have a growing eventual best friend dynamic between Rick and Daryl and the inevitable animosity from Merle because of that. That's good potential story to be told. Between Shane and Merle's wretchedness, Rick and Daryl have a great deal in common.