Just to play devil's advocate here, Mormons have the practice of baptizing people without their knowledge or consent -- even Jews and members of other faiths. People have protested vociferously against this practice (I think it is confined to the dead but don't really know), pointing out that a church that places so much emphasis on the strength of family doesn't seem to be respecting others' beliefs, upbringing, and wishes.

How do mainstream Christians feel in learning that a church whose beliefs they do not share has them on their rolls? Not too good, I don't think.

While I don't subscribe to unbaptising, it's not my business. I'd assume for some folks it's like having been tattooed (spiritually) when they were children and wishing to have it removed. Or changing their names. Just because it's not important to me doesn't mean it's not important to someone else.