Amy is taking me to task here because I deleted one of her comments (however the other one with information about Protestant missions complete with links is still up! I also deleted a post by Kelly and one of my own! Review the commenting guidelines folks!).
I do want to answer this part as it's an interesting question.
While we're thinking about it, why is any Protestant who converted to Catholicism a credible source, but any Catholic who left the church obviously a poorly catechized, 'one foot out the door and the other on a banana peel', lying wreck? Why are conversions TO Rome valid and conversions AWAY FROM Rome not?
I personally in my experience or in my research have never met a fully catechized Catholic who knew their faith and understood it, but still chose to leave the Catholic church anyway for doctrinal reasons. Which is not to say that they probably don't exist, but I think they are extremely rare. I've never met one or read one. It doesn't take very long to find the depth of someone's Catholic education or lack thereof. Which is also not to say that I am not aware of some very knowlegable Catholics who have left, but at least in MY EXPERIENCE they still wanted to in some way hang on to their Catholicity while objecting to the church's teaching on women priests, the teaching on homosexuality, birth control, etc. A notable convert from Catholicism to Orthodoxy, Rod Dreher, did not convert because of he disagreed with Catholic church teaching but because he was sickened by the Priest/pedophilia scandal. Yet even he wrote:
All this takes a toll. And yet, I kept going back to my catechism, and to the truth that none of this undermines the truth claims of the Catholic Church. The Eucharist is still the Eucharist, no matter how corrupt the clerics may be. That was a lifeline for me -- that, and the comfort and friendship of dear Catholic friends, especially good and decent priests, who, aside from actual victims and their families, were probably suffering more from this scandal than anybody else.
Some of my correspondence today has been with Candy's commenters who left remarks on her Whore of Babylon essay. What I learned from these ladies is that they grew up in minimally Catholic homes and indeed were not well catechized.
The soul longs for knowledge of God. When the Catholic parish or the Catholic school, or the Catholic parents don't do the job well that vacancy doesn't go away, it's merely filled with something else. Evangelical Christian churches are great at learning to fill that empty void.
This is the part where I compliment our Christian Brethren.
I might have mentioned that there is a Christian Church behind my house. My kids go over there for the free lunch twice a month as part of the church's community outreach. They also let us have our block club meetings there and they have a neighborhood carnival every summer. And of course they invite us to come to their service and visit their church.
When my sister left the Catholic Church it was because her local Bible Christian Church welcomed her with open arms, got her involved with their activities and made her feel at home.
Years later my sister reconverted back to her Catholicism, but she bemoaned the loss of lively bible studies, kid activities and fellowship at her local parish.
For some reason as we were going through elusive "Spirit" of Vatican II, gracious good manners and being good social neighbors got blown out the door! I think we all know someone who left because Father so and so said something that caused hurt feelings, or some never got over Sister Mary Allen's admonishments to wipe off muddy boots in the third grade. If there is no family feel in the parish, it doesn't take much to make a young developing Catholic feel alienated and want to leave.
That's something we need to continue to work on in our parishes, but also as individual Catholics, at home, work and even in the blogosphere.