He may have been required to do that. Some cars are engineered to where the glass is part of the strength in a roll over.... and good contact between the glass and the seal...and the seal and the car is required....meaning for instance... that it may be required that a specific cleaner is used on the glass immediately before the urethane and seal are applied...
I got this information from a very helpful 3M regional representative. He gave me a bunch of free samples and an inch of tech information on all sorts of 3M stuff......
3M rocks on so many of the things we need for our cars....

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html