I don't think that anyone has suggested that the additional verses have no value simply because they (presumably) were not written by Stephen Foster; just that:
(1)  It's arguable that they add nothing much to the original song, and:
(2)  If they were written by somebody else, it would be useful to know who it was.

Fortunato's comments are at best unhelpful, though no doubt he meant well.  I for one would be interested in any further information that might come up (it's a favourite song); if Auxiris doesn't mind writing to Fi Fraser to ask about her source, I'm sure that most of us would be grateful for that.  

Malcolm