Its by no means an effective weapon like often seen in movies. No knocking baddies off their feet with a single flick of the wrist. It was more used for a distraction and interuption in the attack or flee process. As in, someone closing in on you and you need to make an escape, throw shuriken after shuriken while running away.
As for what constitutes a weapon, well there are two ways to look at it.
One is that it was designed as such. A shuriken, dagger, sword etc are all designed to be used as a weapon. No one designed that thing to open letters...
Two is the act in which you use an object. A screwdriver, baseball bat, framing hammer etc... are not designed to be weapons. But once one is used in the act of violence is considered a deadly weapon in most eyes and will often be prosecuted as so.