Maybe fast travel shown in the demo is for between area's and mounts are good for within an area.
Which works good late in a game when you have cleared out area's
Well in the demo it was definitely within the same area, they went from Redcliffe Village to a campsite all within the Hinterlands. I think it's whatever way you prefer, on foot, on mount or fast travel, but I'm betting mounts are mostly aimed at traveling between fast travel points to areas of the map that you didn't explore the first time through or need to return to for a quest or some such event and no immediate fast travel point is an option. Which also greatly depends on what locations are fast travelable.
I highly doubt we'll see TES levels of fast travel access,rather judging by the map in the demo they be very limited, with only the major towns, camps and keeps that you control or are friendly to you being fast travelable, leaving large areas inbetween where you have to travel on foot or mount.