Comment 2 for bug 1340448

If you try to use more than 95% of the storage, performance will generally suffer -- badly. Now, you may not care for certain use cases; if you are doing backups, you might not worry about that much about performance, and you might care a lot more about using the last few bytes of the disk. But changing this default is not something I plan to do upstream.

In addition for the root file system, you really do want to leave the default at 5% so that root can write to critical file systems. And since the vast majority of Ubuntu users are using a single root file system, that implies that the for the vast majority of file systems created by Ubuntu, the default is in fact appropriate.