If terrorists wanted a plane for a weapon why wouldn't they just get an old, used one. Perhaps steal one from a package carrier. Something like that wouldn't even make a blip in the news. But the Malaysia airliner is under the scrutiny of half a dozen gov'ts and the entire news reading world. If this was terrorists, whatever the plan was has already happened. Those people are gone. Most likely the plane is too. IMO.
By taking a old plane, it wouldn't of generated the news story as you pointed out. Hijacking a commercial plane with passengers on board like this. Created a huge news story around the world. With the details coming out how it flew under radar, changed direction, couldn't be tracked. Plus with how it can't be found and no idea what happened to it. Has created talk and fear of it being used as a weapon. If terrorist did this then that is what they are wanting. Panic and fear in people wondering where it's going to show back up, when will it show up. How will it be used and for what purpose, plus what is their motive. This is also why i suspect no group has come forward with the information yet, claiming that they took it. They really want the confusion and fear to set in. I don't feel you would have that if an old plane thats not used was taken. It would of made local news only.