No, safe spaces don't hinder the development of any individuals. Rather, they provide a break or a relief from what may be an environment that is hostile to their growth and development.
I don't think it's helpful to anyone's development to feel as though they may be beaten, cursed at, and generally treated like they are less than anyone else. Safe spaces give marginalized individuals a place where they are free from that oppression, and also teach those who are not marginalized to be more respectful of each others differences.
Those who contend that safe spaces aren't representative of the "real world" often come from groups that have never been oppressed or marginalized, and have no idea what it's like to live that way. Safe spaces provide some relief from that lack if awareness and empathy, as well.