5% has been the default for a long time, from back when disks were far smaller (tens of megabytes rather then hundreds of gigabytes) so 5% wasn't all that much. Luckily it can easily be tuned down to a smaller percentage as you say, or set to a specific number of blocks if you use tune2fs's -r option instead of -m. In both cases you can give a parameter of 0 to turn the reservation off completely - I wouldn't do this for /, /tmp, /var and so forth, but you might want to for a filesystem that only acts as user storage (say a global file-share) or one that just holds fixed size files (like fixed sized VMs) that will only grow when you create a new one.