Historically, on *nix systems, the reserved filesystem space wasn't intended primarily for performance reasons and has little to do with fragmentation*. The concept was to provide systems administrators with enough space to manage things when the users fill up the disk.

Remember, on [large, multi-user] *nix systems, many files are opened and written to during normal system operation or the login process. This is especially true in a windowing environment (ie., Openwindows, Xsgi, etc.). Without this reserved space, it can be very difficult to login as root and do anything (ie., compress files) to make the system useable for the screaming hordes. When a disk is "full" you still want log files to continue to be written...if login can't write to wtmp, it's not pretty. (Welcome to a good description of my day job!).

With the introduction of "modern," "high-capacity" drives (circa 1992), the historic percentage of space reserved for root has grown to a huge size. I typically cut that down to a small number of blocks, but don't set it to zero. On my personal systems, I'll set the reserved size to zero for "data" volumes, but not for the partition that holds /var, /, or /etc (if they differ).

Oh, by the way, I also run ext3fs on the internal sd and removable sd cards in my N810.

* Since historically (ie., in things like the Berkely FFS, UFS, ext2fs) there's no "defragmentation" command under *nix, so the presence of reserved space still doesn't allow the end user--even root--to defragment the filesystem. I don't consider ufsdump/newfs/ufsrestore to be a "solution".