Thought I might chime in on this one. The unit rosters for all units are theoretically intact(the thoroughness on this varies from unit to unit)for 1943 and before. Rosters for '44,'45, and '46 were apparently discarded. We run into the same issue with records like special orders showing when individual soldiers were transferred from unit to unit. Copies were placed in in the individual serviceman's personnel file, but because of the fire most of these records were destroyed. Unfortunately, many of the original special orders were also discarded by the military. So, in many cases the only remaining special orders showing the transfer of individuals are photographed on microfilm where company clerks inserted them with the morning reports when they submitted them. The issue was really one of space, and not having the room to store all of these records, they were in many cases, burned. Very, very frustrating. Believe it or not the modern military is starting to experience a similar problem today-but with providing DIGITAL space for all of their records! Hopefully, they will not throw away(or delete) documents that will be vital to understanding the experience of our veterans serving today.
I should stress: this does not apply to the morning reports. The reports from WWI through Korea are just fine and were not thrown away-just the rosters for the 3 years noted above. After this they pick up again(into the 1950s).
Hope this helps.
Geoff