while there are a couple spots where capacity is significantly above usage (personal desktops using TB size drives, phones holding audio files), there are MANY other situations where people routinely run up against size limits
think of laptops running SSDs
think servers with multi-TB raid arrays.
in both cases using significant amounts of space for metadata will significantly hurt you.
I have quite a few systems, and many of them do routinly run up against drive size limits. These tend to be personal machines with <150G (SSDs, laptops, high-speed SCSI drives), or servers (where I have multi-TB raid arrays, but they are sized for what I am storing n them)
loosing 30% of the space to metadata would be a problem.