I sort of agree with your friend. I don't think certain names should have nns while others are ok.
Annis, nn
Ann : just use
Ann.
Annis could be made fun of on so many levels.
Belrose, nn
Belle or
Rose : why not just use
Belle Rose?
Felicity, nn
Ellie : one that I think doesn't need one.
Mahalia, nn Halie : I love this name and its considered a future possiblity and I would never use a nn.
Miriam, nn
Mira /
Priscilla, nn Rissa or
Rilla : don't need them either.
Sapphira, nn
Sara :
Sara alone is much nicer.
Asa, nn
Ace : doesn't need it or just use
Ace.
Corben / Korben, nn
Cory /
Kory or
Ben : again, doesn't need one.
Judah, nn
Jude /
Malcolm, nn Mac : again...
I'm not saying that you're wrong to use a nn...just my personal opinions is all. =)