I wouldn't rely on the rafters themselves. With any externally applied lateral force (wind, earthquake, etc), the force needs to be transfered down to the ground / foundation, so bracing upward just means you will have to brace down somewhere else.

If the lateral force is applied to one of the eaves, and the force tried going into the rafters, in order for it to be doing anything, it has to go back out the opposing rafter into the other eave wall, so you've not gained anything. And that is assuming there is a collar tie or something like that.

If the roof is adequately sheathed, then, you may get some diaphragm action where the load applied to the eaves travels in to the roof sheathing, which is attached to the gable walls, and then is transfered down that way. But, this requires a fairly shallow pitch to be effective.

Hope this helps,

Joe