I do this on my instruments, mostly since it permits the vertically adjustable neck that is one of the features I incorporate. Obviously, one can't slide the neck around if the fingerboard is attached to the face!
My fingerboards are supported underneath with a 5mm laminated carbon fiber plate, which extends into the neck from the heel end. This makes the extension extremely stiff, and couples those upper frets to the neck so they don't sound thin compared to the rest.
I agree with Howard et. al that there is minimal (if any) tonal effect - most guitars have a heavy brace up there, which IMO acts as a pretty effective boundary that isolates the neck block area from being particularly active.
However, I also agree with Kent that it is a VERY useful feature to be able to adjust bridge height (and thus, soundboard load) and then dial in the neck to compensate. This is a vital part of my setup process, on a per-guitar basis, and I really feel like it lets me hit the sweet spot for each instrument by making small adjustments to the overall string pressure.
Cheers,
Martin