I don't know much about pins, but usually when the flippers get wonky, isn't it just a worn-out solenoid?
Not usually. Solenoids, or coils don't generally "wear" themselves. Common wear on flippers is the flipper linkage, coil stop (really a plunger stop, but they call them coil stops) and the coil sleeve.
Depending on the game/year, there will be a fiber or plastic connector at the end of the coil plunger, these tend to wear out at the connection points. The coil plunger rides inside a sleeve inside the coil. These sleeves are plastic and also wear over time causing the plunger to be loose in the coil. The stops mushroom over the years from the plunger slamming against it. Any other point of contact can also develop wear. Wear in all these parts adds up and creates loss in the energy of the flipper. Simply put, energy from the coil is lost or wasted in taking up the slop.
Flipper rebuild kits replace all the moving parts in the flipper and bring everything back into a nice, close-tolerance state. It usually doesn't seem like it's going to make much difference, but it really does bring the snap back into old flippers.
Coils themselves usually only fail in a couple of ways. Vibrations from the game can cause cracks in the solder on the lugs causing open connections where the coil windings are soldered on. Coils can also overheat and melt. Overheating is usually caused by something external such as locked on driver transistor, EOS switch problem, mpu board or wiring problems. You can tell when a solenoid has overheated and melted because the plunger will drag in the coil sleeve, or if the overheat was severe, will be stuck.
Hope this helps,
Regards,
D