I needed it today, when I found that the estimated size of a backup was greater than the available space on the drive and had to abort the backup - It works, but it just seems a bit clumsy!
ALSO: I know the estimates are ONLY ESTIMATES, but they're usually fairly accurate - or are they? - I reset my main backup and restarted it:
It correctly reported 19GB to be backed up (2 drives).
Estimated size of backup (after compression) 29GB

The actual size of the backup was 14.8GB.
The next backup (smaller) reported estimates that were much more accurate.

Just one of those computer glitches!
Cheers
Ian