I'm guessing he is just doing the most commonly used joints first.
Yes, the gear joint is a tricky one. I came across that when I implemented an importer/exporter recently. I needed to do them in a second pass once all the other joint types had been given an id to work with. Since Erin is adding to the library itself he will be able to access things a little easier, but it would very handy to have a b2Joint* GetJoint1() and GetJoint2() in the gear joint so that a b2GearJointDef can be fully reproduced using the normal API.
More to the point though, it seems like Diablo's minions may have a little more free time lately... but I'm probably just reading too much into that