To compare the effect of a halter design to a different cut, here's a photo of Barbara looking very shapely in a double V-neck dress:
The silhouette is still luscious and womanly, thanks to Barbara's incredible to-die-for figure, but there's something much more sensual about the halter style. Something about the vulnerability implicit in an open-back style, the fact that it exposes more of the body, creates a certain frisson that (in my observation) men find irresistible.
Still, the above dress is an attractive style in its own right. Interesting colour, too.