First of all, Allan confirmed that the softwrap feature is indeed broken, this has nothing to do with the theme setting.
And softwrap would be defined in themes (although I don't think it currently is) so that you can define it differently for plain text files, where I would typically want it, for code, where I don't want it, etc. So, I think it's a good idea, although it currently is not working very well :(
Jeroen.