That ROM disk is the reason I prefer Classics when going for extra compacts. If I get a Classic without a hard drive or have one develop a dead hard drive, I can always run it off floppies without having to use a RAM disk program (typical for me when dealing with floppy-driven Macs) or disk swaps (which is why I avoid them with the RAM disk; using one isn't always practical if the system has less than 4MB).

I believe the reason the ROM disk didn't exist on newer Macs is because of the size of subsequent systems. The Classic came out in the System 6 era. It's possible to fit a decent installation of 6.0.x on an 800K floppy. For System 7, a 1.4MB floppy can't fit much more than the System and Finder. Apple would have had to put huge ROMs in newer machines if they wanted ROM disk startup disks.