Rod Kronenberg wrote about extended head tubes:
3 very busy framebuilders off the top of my head that do extended headtubes are Rivendell, Sachs, and Kirk. Posts I have read on multiple forums all gush over the comfort and quality of the rides these builder's frames provide. Some may not care for the look, but each builder has far too many satisfied customers for me to believe each doesn't know how to properly size a frame.
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My observation is this, the current trend in component production and the lack of lots of choice in quill stems brings a need in adaptation of the frame design to the components available for some. An extended head tube may provide for some a way to address the dimensional differences of the current parts offerings.
Of course adding an extended height Nitto stem to an extended head tube will provide much higher adjustment than otherwise and still save the vitals upon dismount.
Another interesting evolution is the development of brake levers that allowed for effective riding on the "hoods" while still providing for operation of the brakes, that appeared to influence bar shape, and gave someone the idea of putting shifter controls into the brake levers but that's moving off topic. Pretty clear to me that this change in componentry of the brake levers and bars allowed for changes in the bike frame dimensions.
John Jorgensen
Torrance Ca