Tories probably want the chance to vote for Tristram Hunt.
The party is in a precarious position and they pull this stunt.
'Open primaries' actually have quite a long and successful history, because it forces potential candidates to appeal to more than just the committed base of their party.
Look what happens to the Republican party in America due to their closed system, you end up with rape apologists, people who claim the bottom 47% of the country are parasites, a slate full of candidates who oppose same-sex marriage, etc.
If only the party faithful get a say, that can lead you down some serious dead ends. Of course by opening it up you run into some other risks, but it's not inherently a bad idea or some gimmick.