[Howard Klepper]

Archtops need it (although the less expensive Gibsons had the board right on the top) because of the height of the arch and the height of the bridge. For flattops, the reasons you don't see it much are tradition and a little extra construction cost.

I don't think there is much of a tonal advantage in not gluing the fretboard extension. I glue as far as the end of the neck block in any case, since that is already dead territory. But I think there is some change in tonal envelope from having the strings pull more upward on the top. There is a bit less torque on the bridge and a bit less of the frequency doubling caused by the string tension changing twice per cycle. But not a major change, and easily swamped by other design elements.

The frets over the body are easier to reach, especially for a classically trained player.

Until reading Dekutree's post, I never heard anyone say that their guitar tips forward with a raised fretboard extension. But the neck ends up set at a slightly negative angle to the top (to avoid an overly tall bridge). It is not, however, necessary to have the neck set negatively to the sides. I set the neck square to the sides, and taper the body thickness on the top rather than the usual taper, which is on the back. Builders will get that; sorry I lack digital drawing skills.

The mechanics of it take a little figuring out, but then construction is not more difficult--it just has another couple of steps-- and even has some advantages when fretting and binding the board.