It is important for children to know about their biological parents for many reasons. The trick is knowing when to discuss those issues and how. Most children probably won't be able to handle knowledge of biological parents until at least the teenage years or beyond. Children ought to have a right to know simply because they may be susceptible to genetic diseases they could only get from biological parents.
Some people believe that it is up to the adoptive parent. It's not all up to them. What about the child? Don't they have a say? My mother has wanted to meet her biological parent for years to find out medical history and other important things. But, her parents are objecting though it is a health hazard. My family recently celebrated her birthday. I walked up to her and said "How's your birthday so far mom?" she responded with: "I don't know my birthday, no one i knew was there. This made me very unhappy that my mom doesn't know her birthday. She has the right to learn. She is 44 (or near that age) and her parents still think she doesn't have the right. But, my mother has more than a right to meet her mom.
It is not important to a child's development to know their biological parents. It is important to their development to know that they were adopted and, if possible, the reason why they are no longer with their birth parents. Not knowing their birth parents is probably a blessing, however, if when they are older they have some desire to meet their birth parents, they should be free to do so.