I do sometimes due to cost or if that is the look I want for a quilt like 30 ies. Also I like muslin if I hand quilt.
I have had a harder time finding nice muslin. I buy Moda but it seems lesser quality and have many of the little brown seed shell pieces. I have older Moda muslin I liked for piecing too but I have not been able to get that even if I order the same bolt.
I don't use muslin if I have money or print fabric already that goes. Also my quilting is not very good and the print will be more forgiving.