Along with a couple of the posts above, many classical artists play with a 7 string - the extra string being an added bass string. Many compositions call for a dropped D on the bass, and you either do a lot of tuning in between pieces or you just use the 7th string.
I could be really wrong on this one, but it seems to me that I read or heard where Steve Kaufman, of world class flat picking bluegrass fame, plays a 7 string occasionally, once again for the dropped D tuning on the low bass.
I've seen one classical 7 string model in person - the luthier where I had my guitars made had made one special order for a customer. I commented on how strange it was, and his comment was "not really," so I guess he makes a respectable number (or at least a few).
The only problem I see, (for a guy that has trouble barreing an entire classical fretboard) - what a fretboard width! I think it would be cool to hear one played.