I'll put in my two cents worth here as well (some of this is more or less repeating what others have already said).
If you simply float the fretboard tongue over the soundboard by removing wood from the underside of the fretboard, or attaching the neck so as to leave a gap under there, while not changing anything else about the neck joint or the structure of the guitar, you'll have two problems: First, the guitar may collapse, because the glued-down fretboard tongue of a typical guitar, supported by the big transverse brace in the upper bout, is part of the structural integrity of the guitar, i.e. how it holds up to string tension. Second, any notes fretted above the body joint will sound dead because the fretboard tongue is not rigid enough by itself. Hence, designs like David's and others, which support the fretboard tongue, and also -- not shown in David's pics, but I'm pretty sure he does this -- provide greater internal support for the neck block with buttresses of some kind, so that the neck/upper bout structure is rigid.
Personally, I believe that joints like this are well worth exploring and pursuing for all the reasons that have been stated: First, separating the structure that supports the neck from the soundboard, so that the soundboard can be braced for optimal sound and more of it's area can be actual soundboard. Second, allowing for an adjustable neck joint, which is, in my mind, clearly the best way to make adjustments to action. Third, allowing for the elevation of the fretboard and changing of the neck angle in relation to the soundboard -- guitars made this way may not sound "better" to any given set of ears, but they do seem to be louder and to have desirable acoustic attributes.
I'm working on my own design (very similar to some others, notably Gary Southwell's) on the two instruments I'm building right now. My progress is very slow due to limited time right now, but I'll post some pics when I get to this construction.
I'll throw in my .02 on McPhersons as well. Haven't played 'em... my comment is on aesthetics. I applaud them for their innovation. I'm always surprised by how conservative so many people are. I don't happen to like their design very much, visually, but not because it's different from traditional guitars. I don't really like the aesthetics of many traditional designs. It's all subjective, but in my opinion, many guitars' shapes and overall aesthetic designs are ill-conceived. To my eye, McPherson didn't get the curves right, but neither did Martin when they came up with the Dread or the OM. Now, the OLF SJ, THERE'S a gorgeous shape if you ask me -- my hat's off to Michael. I also like several of the more unusual shapes designed by makers (both historical and contemporary) with unique visions and the courage to be original, and would much rather see more of those than more of the same old same old. Hope I haven't ticked anybody off... just my opinion.
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