Apparently debating the merits of Mac OS 9 is very 2014. Via TenFourFox Development, all the hubbub was kicked off by an Ars Technica article* about living with OS 9.2.2 for a few days, which was followed with a rebuttal from Riccardo Mori at his blog. Some previous points of view on the subject are from The Vintage Mac Museum and LowEndMac, but I wanted to use this as an excuse to point to a Mac OS 9 Lives Forum thread which reveals how to boot OS 9 on MDD FW800 Power Macs.
(UPDATE: This note was left in the comments: Flashing the G4 FW800 MDD is no longer needed and it is not the preferred approach, we now have a Bootable 9.2.2 Install CD (with Modified ROM 10.2.1) that will boot unsupported G4s and Install 9.2.2. You can get the CD ISO image at www.macos9lives.com/smforum or at www.thinkclassic.org)
First sold in 2003, the FW800 models were the first Power Macs, and only G4 Power Macs, to not boot OS 9. This remained the case till over a decade later when this Mac OS 9 Lives thread, "Downgrade firmware of FW800 for OS9 comp", appeared. The whole thread is incredibly long (these were a dedicated, persistent bunch), but I'll just point to this post which has the actual solution. Basically it involves flashing the firmware with an older version that supports booting into OS 9. This works for the FW800s because the earlier firmware is for a very similar MDD model.
Efforts are also underway to duplicate this approach with other Macs, and the thread, "Mac Os 9 boot on unsupported iMac G4", reports limited success. Perhaps this won't work so well on other models because the only available firmware downgrades are too old and for too dissimilar hardware. It sure would be nice seeing an aluminum PowerBook booting OS 9, though.
As far as I read, the only limitations on the MDD are the disabling of the Firewire 400 ports in OS 9 and the Firewire 800 port running only at 400 speed, also only in OS 9. In OS X everything works fine. But for that, you get an OS 9 booting machine for your FW800, and now you too can join the online Mac OS 9 wars! Small price to pay for having access to all that great audio, productivity, and gaming software from the '90s-'00s.
*WARNING: That Ars Technica article is like weaponized banality.