The top of the pad still has some material left on it, so should I have let the pads go longer? No. Look at the second picture, below. This focal plane shows more clearly that the pad has not worn evenly, to the point that the copper "panhandle" that sticks out of the caliper has obviously been hitting the rotor during braking. Oops. So yeah, it was time to change the brake pads. (Both of these images are from a single picture from a Lytro camera.)
Measuring with a caliper, the top had about 1200µm of pad left, and the bottom had at most 300µm left.
Interestingly, the inner pad is worn completely differently. The inner pad wore evenly top-to-bottom, but the front edge had had 950µm and the trailing edge 1100µm. So the inner pad wore almost flat by comparison, except the leading edge wore a bit faster.
Why did the pads wear unevenly? Well, I'm not sure.
Maybe I didn't set the brakes up correctly? BB7s mount with spherical fixing bolts, so that you can align the caliper in two axes with the rotor. Avid's old instructions and Park's current instructions for this are laughably incomprehensible, so I figured out a simple way: put in new pads, set the pad distance. Squeeze the caliper shut on a trued rotor. Tighten the fixing bolts. So maybe that's wrong? I checked the Avid site. Avid's new instructions are pretty much exactly what I did. So that's not it: AFAICT, the caliper was set up with the caliper coplanar with rotor. There isn't much play in the caliper inboard and outboard, but even if there were, the caliper is designed to compensate for that by adjusting the pad positions. This is necessary, since hubs vary in exactly how outboard from centerline they mount the rotor.
Maybe I had let the pad positions get out of adjustment? The BB7 has two knobs to adjust the pads inwards as they wear. The outer pad is on a piston that pushes it towards the fixed inner pad. Obviously, the rotor makes an angle as it's bent, and obviously the force of the rotor is going to be higher on the top of the outside pad and the bottom of the inside pad. But that's the opposite of the wear pattern on the outside pad, and the inside pad wore evenly. So that seems unlikely.
Maybe the pads aren't coplanar? I doubt the caliper body is bent, since it's so stout that I find it hard to believe a blow hard enough to bend it wouldn't throw it completely out of alignment. The piston that pushes the outer pad in obviously has more play in it than the fixed holder on the inner side, so maybe this is just as good as it gets. One way to look at it is, I got 3 winters off of a set of brake pads that cost me less than $20. (The rear pads will probably last a couple more winters yet.)
Hmm. If you have any experiences or ideas, or interesting things to measure, I'm all ears.