My adopted sister's birth family was in and out and it was nice because she didn't have linger questions about who they were and the reasoning behind them giving her up. They weren't a threat just like extended family members basically. I think that is how many parents with open adoptions feel that it just goes to enhance the kid's life.
I have a good friend who was adopted as well and her's was closed. She has no answers about her medical history (which is now proving to be a problem for her). She has no clue about her heritage. She only understands the circumstances of her adoption a little bit (her birth mom was very young and gave her up). But she has so many unanswered questions. She loves her adoptive mom but the adoptive mom can't give her the closure and answers she needs and the adoptive mom knows that and is equally frustrated .