Considering they knew exactly how Sin was reborn all of the time, and killed the thing that kept bringing it back, bringing back Sin is a terrible idea. They might as well just say to fans that they don't have any good ideas of where to go with the story, so we're just going to bring back the things people liked whether it makes sense or not. Because who needs good ideas when we can just rely on nothing but fan service?

Then again, we're talking about the people who wrote X-2 here so there's no reason to expect anything good.