This option should check if there is enough free space in drive for the operation BEFORE making any change to it. I needed to use it and it had no enough free space. It aborted afetr some time and I lose ALL data. (This could be avoided by adding checks before doing anything...)
Gentoo bugzilla is a wrong place to ask for features. That needs to go upstream. http://www.namesys.com/
http://www.namesys.com/