I always thought another reason for the preference was that you got an extra 1"
or so of interior height. Is that wrong?
Dennis Storzek wrote "ZU eaves were preferred on newly built insulated cars, because all the roof fasteners are external, and a roof can be repaired while the car is under load. It wasn't mandatory, however, and some RB's had the normal roof panels applied to Z bar eaves. The trade off was the car had to be unloaded and the ceiling opened up to repair a damaged roof."
Dennis your comment is the first time I recall reading why there maybe a preference for "ZU" eaves.
The B&O started using "ZU" eaves on box cars, non-insulated, with the kits ordered from ACF and P-S
for their class M-66 and M-67, respectively.