A soft focus lens can work progressively, keeping some areas of the picture sharp (or sharper) than others. It can also defocus the foreground or background more significantly than you moght expect for a given aperture. A soft-focus filter cannot... it just mists over the whole image.
For most people, the soft-focus filter (or doing the same in post) is all you need. Those who need a soft-focus lens have no problem justifying the cost, because it's a pivotal part of the photography.
Many thanks for your quick and informative reply. I think you have totally answered my question. I will put the soft focus lens on the long term wish list and stick with tweeking with Photoshop as it does the job pretty well and can be easily more selective.