I have been crazy quilting for about 15 years, and have always used muslin. It adds a stability when using different fabrics and helps hold them together...a friend of mine made a pillow without the foundation, and it looked fine, but when she took it to a seamstress to make the pillow,(she didn't know much about sewing), she was told to add the foundation after any way to add stability before the pillow could be made! The pillow came out nice, but what a hazzle! I just fine it easier to use a foundation. When doing something like a vest, it gives you an idea if it will fit, and where to place your fabrics.
Just my input.....