Probably stating the obvious but it is worth noting that some outsoles are sewn using an "open" channel--a groove, really. It is slightly better than stitching "aloft" (no channel, no groove) but not by much. It doesn't protect the stitches much at all...allowing immediate exposure to grit and moisture. But it is the most expedient (cheap) way to approach the problem. The sewing is done with a machine.
A step up would be a vertical channel. Several types of machines will make the channel as the sewing is done. The channel can be closed and the stitches are well protected. And there is no real risk of a loose or flapping channel. The closed vertical channel is only barely visible and requires no glue or cement.
What is shown in the photo is a different kind of channel--one that has also been cut by a machine. It can be done by hand and sometimes is, but esp. for production work, such laborious attention would be impractical. When done correctly the outsoles can be sewn by a machine as in BS's photo and then the channel will be glued down. The channel cover is usually pretty thin and the glue is usually some variation of wheat paste, potato flour or even animal glue. These glues are not stable in moist conditions and when the channel cover is laid back down the leather is often moist. The stitches are nigh on invisible initially and well protected, but not for long...as can be seen.
The typical hand cut outsole channel is cut at an angle from some point close to, or at, the edge of the outsole. This is a very good solution but is the most time intensive. The outsole is generally sewn by hand and the channel closed by hand. Because the leather covering the stitches is much thicker, the channel tends to stay closed. Glues are used as a second or third line of defense rather than a first.The stitches are well protected, entirely unseen, and all other things being equal, the hand cut, angled channel will afford the longest wear.
Even so, eventually all methods leave the stitches exposed as the leather wears away.
A good leather cement (there's a difference between traditional glues and cements) will remedy the problem. Superglue will work but it is overkill and will stiffen the leather and make it brittle.
Edited by DWFII - 12/26/12 at 7:13am