Originally Posted by white lobster
Bingo.
I've heard pro mechanics complain that some old cartridge-style brake shoes could actually come loose if you slapped a wheel in too quickly and hit the pads with the tire. Then you had an extremely ticked off bike racer without a functional brake.
Those metal tangs that point down at an angle guide the wheel in and the rubber plugs keep the tire from banging into the pads. They're rubber since there's a chance that they could contact the rim if the pads wear out and you wouldn't want metal-on-metal.
Bingo +1
The wings make a good guide but without the screws slamming the wheel into place can still knock the pads out of place leaving you with no brakes.
As for being wear indicators for the pads, I'd be really pissed if that screw ground into the sides of my expensive rims.