By default ext2 and ext3 filesystems reserve %5. While this is useful for /, /tmp, and /var, this wastes precious harddisk space for /home, and since /home is probably the biggest partition users create, not reserving %5 on /home, or reserving %1 returns something like 2GB of harddisk space that is unusable otherwise...
The reservation isn't there solely for the purposes of not filling up the disk. ext[23] also needs that reserved space to avoid particularly bad fragmentation.
Why must your preference be the default? I'm it has a useful purpose.