I have played a couple of "harp guitars" by UK maker Dave White.
http://www.defaoiteguitars.com/page10.htm
In particular, the baritone harp guitar (at bottom of page linked to above) is a stunning instrument with potential capabilities limited only by the players imagination and sill. Have a listen to the soundclips at the bottom of the page. As with many harp guitars, the low, unfretted strings are best used, not in any particular fixed tuning, but tuned as appropriate for the piece to be played.
Even when not playing the "extra" strings, the sound from the "normal" fretted strings is hugely enhanced by the extra soundbox capacity and by sympathetic resonances from the "extra" strings.
I wouldn't want to own one, but they are serious instruments, and in the right hands can produce some lovely music. Very far from pointless!
Keith