Not sure why but we probably won't enable it as
a) Turning on progress bars for the initial job (not the duplicate) is NOT recommended because it adds a potentially long scan and calculation time to the job that means your backup window must be extended. This is why it is not on by default (Note I assume you have turned this on and do see progress bars for the initial jobs?)
b) in reality how many people watch the progress bars for backup jobs which often run at unfriendly times of the day and could take hours to run as well
Yes, of course the prescan adds time to job, but even on 5TB fileserver backup it adds only 10-15min (actual backup time is about 1 day + 2 hours), so it acceptable. so we keep progerssbars enabled. It's very useful because we can make decision when we need to do something with backup server or a server is being back up at the monent - do it now (and abort backup) or wait until it done.
The questions is, "in the case of duplication, the data size is already known, so prescan is not needed, so why turn them off?"