I don't have a ProTools setup, but for me, there are two main considerations.
First is that there is a lot of software which only runs on the older MacOS. I don't usually get very nostalgic about computers or operating systems. But I make a point to run whatever will host the software that I like, and allow me to be productive. For people who mainly use popular software there is less of a need, IMO, because there is some version which works on either OS 9 or X. But most of what drew me into OS 9 were weird quirky programs and plugins which were one-off projects, so I run an OS 9 system, or I don't get to use that stuff.
Secondly is that OS 9 can be very responsive. This makes it hugely forgiving of more modest hardware, as can easily be demonstrated by dual-booting even the best G4s with OS 9 and X. The older MacOS are basically a real-time, low-latency system, which can be great for audio and MIDI work. But there are tradeoffs, such as poor multitasking, poor security, and especially lack of protected memory. So these systems can be less stable, and take some finesse to keep them running well.
I don't know if running these old systems is worthwhile merely for the sake of doing it. But there is a lot of software out there which one can be quite productive with, and have a lot of fun. Also, to some extent, the fact that older Macs have more trouble on the web and watching HD video can also be beneficial for concentration, because there are less distractions from one's projects.