I agree with you in this one. While I admire the fact that Labour are trying to do something about it, it doesn't seem as though their plans will do much about it. Like you suggested, companies could easily get around the 12 week rule. In which instance, Labour are not actually 'banning' zero hours contracts as they suggest, because they could still exist for a period of 12 weeks. Hopefully there will be a solution to this problem in the future.
Edit: just to pick up on your last point though, I don't think they are showing themselves up to be as bad as the Tories on this issue, who haven't even mentioned it as a problem. On the contrary, the Tories use the boost in employment figures which zero hour contracts have brought about in order to gloat about their employment figures.