I don’t understand how you believe that agencies coerce pre-adoptive parents into openness. Just because you don’t like the options available to you doesn’t mean you are being coerced. If you go into a vegetarian restaurant and are not able to order a hamburger, are you being coerced to change your eating habits?
If you don’t like the match that’s presented to you, either because the desired amount of contact or the values of the expectant parents are not similar to yours, DON’T TAKE THE MATCH. Wait for one that works for you or move to a program that offers more opportunity for closed-ness, such as other posters here have done.
Evidence, anecdotal and otherwise
Anecdotally, you say, see the boards?? OA is a disaster because so many people complain about it. Anecdotally, I say, I know of lots of families making OA work. I say half-full; you say half-empty. So what does empirical evidence show?
OA is still fairly new and, as you point out, the children of the earliest are just now coming of age or are young adults; hence the dearth of studies available.
However, here is one article about a study funded by the National Institutes of Health:
Leve [Researchers Leslie Leve, Ph.D., and Jenae Neiderhiser, Ph.D., are among the principal investigators in the Early Growth and Development Study, funded by the National Institutes of Health] theorizes that both birth- and adoptive parents become more comfortable about the adoption as time passes, and with greater comfort comes a desire for more contact.
But Leve also ties this desire to larger, more universal trends in our society. “Individuals who have participated in adoption plans are likely no different from those who haven’t, in terms of wanting more contact and a better relationship with extended family members, biological or not,” she says. “The dispersed nature of our society prevents many of us from having as much contact with extended family as we want.”
And there’s this:
In fact, the researchers found that openness significantly correlates with satisfaction and post-adoption adjustment among birth and adoptive families alike.
“You can add openness to the list of things you don’t have to worry about,” says Leve. “It almost always works out.”
She explains that birthparents and adoptive parents select the amount of openness that fits their comfort levels. “A birthmother who wishes to remain anonymous and have no contact with the adoptive family is not going to seek services through an agency that advertises itself as promoting openness.”
Neiderhiser, herself an adoptee, endorses an attitude of openness to all adoptive parents. “The more you talk with your children in an open, positive way about the fact that they were adopted, the less of a problem it will be for them.”
Findings of the MN/TX Adoption Research Project (are you reading, researchers? We’re ready for your latest data from Wave 3, underway from 2005 to 2008) include these regarding the adoptee :
…adolescents demonstrating integrated adoptive identity had coherent, integrated narratives in which adoptive identity was highly salient and viewed positively. For example, one teen said, “When I was little I worried I was placed because she didn’t want me. Now I know I was placed because she cared enough.”
Adolescents having contact and expressing satisfaction with the contact (45.5% of the sample) stated that the contact provided an opportunity for a relationship to emerge that would provide additional support for them. They also expressed positive affect toward their birth mother, felt that the contact helped them better understand who they were, and made them interested in having contact with other members of their birth family, such as siblings. Adolescents having contact but not expressing satisfaction (16.3% of the sample) typically wanted more intensity in the relationship than they currently had, but they were not able to bring it about. They felt that they could have good relationships with both adoptive and birth parents, and that they did not have to choose one over the other. Adolescents not having contact and satisfied with the lack of contact (17.1%) felt that adoption was not an important part of their lives. They did not feel that it was necessary to have contact, sometimes expressing concern that contact might be a bad experience for them. They felt they were better off where they were (in their adoptive families) than they would have been if raised by their birth parents. Finally, adolescents not having contact but dissatisfied with the lack of contact (21.1%) sometimes desired contact but were unable to bring it about. Some had negative feelings toward their birth mother or assumed that she had not made an effort to have contact. Some worried that their adoptive parents or birth mother might feel bad about their pursuing contact. 
Birthmothers who were older at the time of placement were more likely to be satisfied with their current openness arrangements at Wave 2. At Wave 2, birthmothers who were older at placement also felt closer to the child’s adoptive mother than did birthmothers who were younger at placement. Most birthmothers reported feeling positive or very positive about their relationship with the child’s adoptive mother and father and were satisfied or very satisfied with these relationships. At the same time, the majority of the birthmothers indicated that they had at least some concern about whether their contact or potential contact interfered with the adopted youth or adoptive family functioning. Almost 20% were “very concerned” about this issue. 
and this regarding adoptive parents:
At Wave 1, when compared to parents in confidential adoptions, those in fully disclosed adoptions generally reported higher levels of acknowledgment of the adoption, more empathy toward the birthparents and child, stronger sense of permanence in the relationship with their child as projected into the future, and less fear that the birthmother might try to reclaim her child. 
Regarding your concern about the lack of public representation of adoptive parents on the Internet:
That made me laugh (not in the funny-ha-ha kind of way) because that’s exactly what I’ve seen birth mother and adoptee groups say about themselves — each feels underrepresented, voiceless, powerless.
Further, members of birth mother groups have said that adoptive parents are the only “winners” in the game because they are the ones who end up with the baby and have all control over relationships. Members of adoptee groups say they were the only ones who don’t have a say in the matter of their adoptions, so they are the true victims.
No one has a lock on being the oppressed party.
If we keep beating, the horse will surely die
Clearly, I am not going to change your mind any more than you are going to change mine on these issues. I don’t even belong on this thread based on its title. I did not join the conversation until later when it became about me.
I am happy, though, that a counter viewpoint is here in case others who are on the fence about openness explore the ground we’ve covered.