I'll admit that I haven't been able to find any corroboration for my 'deadly as a hail of machine-gun fire' statement, but I HAVE found one or two stories like the following on sites dealing with these subjects:
Saito Sensei apparently had a few people around town who didn't particularly like him, because of either some property deal he had negotiated for O-Sensei, or for his role on the town council. One night during training, a couple of tough guys turned up to the dojo, calling Saito to come out. Saito ignored them, so they stepped into the dojo. Saito apparently also had a reputation around town as a shuriken guy, and these guys must have turned up during class thinking he would be unprepared. Saito didn't even look at the guys as they came, didn't even acknowledge that they were entering the dojo. But he just slighly pulled open his keikogi a little bit, enough to show them the three shuriken sticking out of his obi. It seems the guys weren't so tough after all, and they turned and ran out the door.
The clear implication being, it seems to me, that Saito could have done some serious damage with those shuriken if he chose, and they knew it. 'Shuriken guys' were evidently fairly respected in battle. Furthermore, while most sites do agree with you that the traditional use of shuriken was mainly as an irritant in battle rather than as a deadly weapon, they usually go on to state that the reason for this was that Samurai warriors, who were the usual targets, typically wore thick padded armor into battle, meaning that it COULD only be used largely as an irritant, because the shuriken would stick in the armor, only piercing the skin shallowly if it pierced at all - however, if it hit in a vulnerable unarmored spot such as, say, the throat, you could be seriously injured or killed. Your average person does not generally wear thick padded armor, and therefore would be far more vulnerable to a handful of shuriken. FURTHERMORE furthermore, there is, in fact, a specific martial arts school of shuriken throwing, called 'Shurikenjutsu', the details of which are foggy, as they have been kept a closely-guarded secret ever since the shuriken were first created. I would suggest that there would be little point in such secrecy if shuriken were not, at the very least, an effective tool in battle.