I never really thought of collect-a-thons as "adding game play."
Why not though? For me, at least, I always enjoyed exploring the different areas in a game. Plus, even if you don't have to, collecting things is fun. Call me strange, but I think it adds to the replay value and give more life to the game play. After I beat OoT, for instance, I remember looking around Hyrule for all the pieces of heart. Yeah, I'm weird like that.