@Iluviaporos - I think it depends on how good you are at innovative sewing and how well tailored you need the dress to be. This is a profession for a reason and people who just sew for a hobby aren't going to be able to complete a dress to the same standard as a professional designer.
If you want to design your own wedding dress, for example, you aren't going to want your mother to do it unless she is a professional. If we're just talking about costumes for a school play, that's one thing, but if you need the dress to be of good quality you don't want it done by an amateur. And a professional designer will be able to make it look the way you want it to, as well as function properly as a well constructed garment.