Leah Day is a pro and has books, classes, and a huge following. She probably knows good reasons for two machines from experience. For me it's because it s a pain in the butt to set up one machine to do both. Change the feet, change the needle, change the tension, test sew, before I can start sewing. I have a Bernina I use for piecing but won't use it for machine quilting even though it does a great job. I have a Brother 1500 I use for quilting but won't use it for piecing even though it does a great job.