My grampa (born in Kentucky during the Civil War) did the same thing. I think these may well be holdovers from English customs of a couple hundred years ago. Georgette Heyer was - in general - meticulous about the accuracy of social customs in her historical romances, and I can recall scenes in at least one or two of her Regency novels where people poured their tea from the cup to the saucer before drinking it. Not sure whether this was a custom which originated in society and trickled down, or of the common folk which percolated up.
I don’t know about you, but when I’m dealing with hot soup, I use a spoon.
I too, had a relative who poured his coffee into a saucer to drink it. He was born in Georgia in 1902. In his case it was definitely not a remnant of upper class or English heritage, as he was born into a share-cropping family, of Native American and Scot descent.
The majority of the latte cups I’ve seen in American coffee houses are pretty much just bowls with handles (excepting the ubiquitous paper coffee cups, of course).
My grandfather was of German descent but his bacground was very rural, poor, and from central Missouri. It’s possible that it’s just “common sense” to do this and without people around to tell you that it was improper, you did it. It seemed like folksy smarts to me at the time.