I have seen quite a few old bags with "flat stitched " seams..both bound, and unbound...some done this way are pretty plain and though charming, are border line crude...but the majority are finished very neatly.
At 7 stitches per inch, which seems to be a good average used in the old bags, it can take about 500 stitches to make a standard double bag with a bound flap edges...and flat sewn they will all show...that many well done "exposed" stitches are almost a decorative element of the bag...I dont know if it was purely function and design, or if the old makers liked to show how well they stitched ... maybe both?
TCA