I think that the plunger seal is only used for keeping the pressure in the fuel system when the car is turned off. When you turn off the car the plunger sits on the seal and keeps the fuel from leaking.
When the engine is running, the rubber seal is useless because it lets fuel through the plunger stem area.
There is a theory that the fuel distibutor leaks because the plunger itself gets thinner by time letting the fuel sneak through its walls. But it is not proved yet.
Hopefully we will have an answer from a professional.
I would recommend you to take a look at this site.
http://www.injectorservis.narod.ru/KE-JetronicClean.html
This russian says that he has fixed some bad fuel distributors by taking them apart and then cleaning them in an ultrasound bath.
No info about leaks but this is usefull information.
Here is a pic from his site.
http://www.injectorservis.narod.ru/KE-JetronicCleanFiles/DozatorUltraSound.jpg
He says he can fix most of the bad fuel distributors. The second picture shows us that he takes a measure of heights of the 4 screws. After the ultrasound bath he re-adjusts them to their original hieghts.
Does anyone want to take their salvaged fuel distributors apart?