I'll test other DOSes to see what happens.
704K.COM just tests to see if there is memory at A000; if there, it adjusts the BDA and then warm reboots. So, what happens on various DOSes? On my 5160 with 64K at A000, and an empty CONFIG.SYS file:
MS-DOS 3.3: Successful, 651K free RAM after clean boot and running 704K.COM
MS-DOS 6.22: Successful, 642K free RAM after clean boot and running 704K.COM
DR-DOS 6.0: Produces erroneous error message "BOOT Diskette defective. Press F1 to retry or any to continue." after 704k's reboot. (F1 repeats message, other key goes to ROM BASIC)
PC-DOS 7.0 Revision 1 (IBM PC DOS 2000): Locks up the system after 704k's reboot
So I'm not sure what the unsuccessful DOS versions are doing, as 704k.com is extremely simple and doesn't do anything wacky. Maybe the typical warm boot process (set the BDA flag, jmp to fff0:0) doesn't work on all DOS versions, perhaps.
Why is an additional 64K lower DOS RAM appealing to me personally? I like to code in the Turbo Pascal IDE, and it doesn't use UMBs, only EMS. While I have an EMS board installed, it doesn't free up all that much lower DOS RAM when using the TP IDE. An extra 64K would mean I could debug my larger projects on the 5160 itself.