I'm not sure what Harriet's daughter's work says about this topic, but I do know that there are many people here on the boards who work with poly thread without any problems, which lends credence to the Superior Threads people's opinion. And what is said in the Superior Threads video makes sense - good quality fabrics are less likely to be damaged by any thread and cotton thread can be stronger than poly thread. That all taken together I think using just cotton might be overcautious.