I don't like it at all, never have. It just seems so snooty and prep-school to me, like her best friends
Mallory and
Meredith.
I also dont' see why
Hilary would be any less likely to get shortened than
Hillary. They're pronounced exactly the same.
I think Hilary/Hillary might have become more popular if it wasn't for
Hillary Clinton. She was pretty much the only famous
Hillary at the time, aside from some soap opera character, maybe. And she wasn't young. She kind of dated the name and rendered it middle-aged even though when she was born it probably was considered cutting-edge.