No one puts much stock in this outdated idea any more. I think you are right socially, but as a couple of people have said, 'illegitimacy' is actually a legal construct, and I would not be in the least surprised to find that the law has not caught up with the social in significant ways. It might be taking too much of a risk to assume everything will work itself properly without a will by the parents, for example, especially if they have some legitimate and some illegitimate children.