I tested this on the PTR this morning. Warriors do gain fractional rage from attacks - if we take less damage than is necessary to give us a rage point, it counts toward the next point we would gain, so we still gain full rage from low damage attacks. However, if the fraction falls below a very low threshold, we don't gain even partial rage from it.
I walked outside my garrison and got in a fight with a bunch of mobs that dealt about 0.1% of my health per hit. After letting them beat on me for a while, I was down to about 70% of my health and had gained 0 rage.
I then fought some mobs that dealt about 0.5% of my health with each attack. After falling to about 70% of my health, I'd gained 15 rage.
So unless we're taking really trivial damage, we appear to gain full rage from attacks that don't deal enough damage on their own to grant us a rage point.
Here's a link to the Imgur album with the above test images.