I think it's really more a carryover from the old days when UNIX was mostly used as a server. The issue was that you didn't want some rogue application to completely fill up all disk space and leave the machine unable to do anything. Thus, the reserved space allowed the superuser some room to clean up stuff if needed.
These days with Linux used as desktop machines, it's much less necessary to reserve so much space. I've cut down reserved space on some of my 100GB partitions to 1%. Here's an excerpt from
Optimizing Linux filesystems
Ext2fs and ext3fs reserve a number of blocks for use by the superuser (or some other user you specify). The default value of 5 percent reserved space may be overkill on large partitions or on less critical partitions (such as /home). You can gain a bit more space by using the -m reserved-percentage option to mke2fs. Changing this percentage won't affect actual disk performance, but it may gain you just a bit more available disk space. You can change this option after you create a filesystem by passing the same parameter that mke2fs accepts to the tune2fs program, as in tune2fs -m 1 /dev/hda4 to set the reserved blocks percentage to 1.