This is what I'm seeing. The satin canvas I guess allows you to use the photo black ink, which is important for the guys who have printers that don't easily swing between the blacks. So if you already have photo black loaded for printing on glossy/luster papers, you don't want to have to make changes for going to canvas.
But I am seeing that its my coating that determines the final sheen, not the canvas. I have also found terrible head strikes with the satin/glossy canvases since the edges curl up as the printer prints, and as you say, the matte is cheaper. So there really isn't much of a reason to go with something other than a matte canvas I don't think. I mix my timeless in a ratio of 20/80 gloss/matte for the matte look (it needs a bit of gloss to just make it stick better or else is has this talcum powder feel), and for a satin/gloss finish, i use an 80/20 mixture of gloss/matte.