Reading the feat text in the PHB I agree that it does spcifically state multiple "arrows" fired at the same time. This is understandable as the archer is holding several arrows drawn and releasing them at the same time. However, I would allow a PC to use the feat for Throwing Daggers, Shiruken, or other similar weapons in conjunction with the Quick Draw feat. It's easy to imagine someone thowing several such weapons by drawing them from a bandolier, or a bracer on thier opposite arm. Just my opinion. By the RAW though, its definitely arrows only (I think it was to avoid the non-sensical "several bolts loaded in my x-bow at once" shot, which would be possibe w/o the arrow-only stipulation).