In this case, I believe that the gear is weaker because it is missing a substantial amount of material in the middle; it is a "doughnut" or ring gear instead of a solid single piece gear. The reason why I think this is because none of the teeth on what is left on the gear are stripped or look like they are about to be stripped off.

The helical cut teeth are superior to a straight cut gear with the exact same tooth thickness along the gear pitch. The reason why a straight cut hear will be stronger, assuming the very same metallurgy, is because the teeth are bigger with a larger pitch to accommodate the thicker teeth.

Here is an illustration from Wiki (There are better sites that describe gear design, but I was too lazy to search more in depth):
http://en.wikipedia.org/wiki/File:Gear_words.png


Based on no missing teeth on the gear (at least what we can see of it), this causes me to think that the fact that this gear is a ring is the cause of failure. The material missing in the middle (the "doughnut" hole) therefore is structural support material that does not reinforce the gear ring. And the ring is now able to twist and bend sideways more easily as the engine's torque transmit forces onto the spur. And combined with one or two screws missing from the cross piece gear adaptor, the gear ring is allowed to twist and bend even more; to positions beyond the failure point.