The primary advantage I have found to half soft frost is that it's very efficient in terms of how much light is lost for the degree of softening you get. It softens a similar amount to opal or 251, but it doesn't cost you as much light. It's also quieter and less prone to tearing than opal, if that's a concern in the given situation. A drawback to HSF is that you can't use it clipped to the barndoors of tungsten or HMI because it'll shrivel and melt. Also notable is that it can be both welded and sewn, therefore making large rags a possibility, unlike opal or 251. I guess weldability is more my issue than most people's...
DP, gaffer, inventor of Airboxlights.com Inflatable Softboxes
SF Bay Area