Being a unix newbie, I took the advice of a friend who claimed to know all about Unix, though I really knew she didn't, because her ex-boyfriend was a major computer guru. When I was having some small problems with OS 9, early, after 10.2 was released - she recommended that I simply reinstall OS 9.
I did that.
It didn't cause me much problem in OS X. I'm pretty sure files are messed up, but OS 9 runs fine in OS 10. At least for my regular use. I have found no particular problems there.
The real problem comes when you want to boot up in OS 9. I don't need to do this anymore. However, I have an older Powerbook. When I did this, I had the hardware based DVD player in my Powerbook, and you could not use the OS X DVD player with that DVD player. So, in effect, I gave up playing DVD's. Since then, I transferred the hard drive to a reconditioned old powerbook with the software based DVD player. I can watch DVD's in OS X now.
The point of this is, it probably will cause you problems in booting up in OS 9 directly, but not using classic within OS X. I would guess that the reason for this is completely security related. If a hacker could simply install OS 9 to get complete access to your computer - then that would be a serious hole in the security of OS X.
I still intend to fix this, but backing up is also an additional wrinkle with the additional file information that needs to be copied when critical system files are backed up. I have to buy one of the back-up software programs - I'm not going to use the various scripts printed here - I simply am not that confident about my unix skills.
I hope this was vaguely helpful with regard to this question.
