A Mom Blog Social Network
afterbedtimeblog(dot)com sounds a heck of a lot cooler than afterbedtimeblog.blogspot(dot)com. Your traffic stats, is your traffic, not Blogger's and will travel with you if you leave Blogger.
Having your own URL adds professionalism to your site.
Registering your URL encourages commitment to your blog by nudging you to be committed because you've invested money in it.