OS 9 is a wholly separate entity. However, if you want to use the classic environment (and even if you aren't now, you might have cause sometime or another) you need it. So unless you are serously running low on hard drive space, I don't really see the harm in just leaving it there.
Why on earth would you want to remove OS9 for, anyway? Here's some reasons to keep it:
It's great for games (much faster in some cases than OSX)
Hard Disk space is ridiculously inexpensive, so there's no point in taking it off
If you've got a partitioned drive (like me) then you can put OS9 on one partition, and OSX on the other (I have a 20Gb drive, with 5Gb for OS9, and 15Gb for OSX). This is great for when your drive goes down - at least you have 2 lots of boot sectors, so you'll definitely be able to start your computer to repair the damage
Not everyone is a genius when it comes to understanding Unix - and when the thought of using Terminal breaks you into a cold sweat, and you don't know how to use the "sudo - " command when you are desperately trying to remove stubborn files from the Trash, don't panic. Boot into OS9, and empty the trash as usual. Problem solved.
Classic is an integral part of OSX - removing OS9 will stop this application from running. It's still important because some installer programs still require the computer to boot into OS9 to complete the installation process.
There are STILL some applications that are not yet carbonised.
I'm sure there are other good reasons, and there'll be plenty of suggestions from you all as this thread progresses.
Having used both seperate and in tandem quite extensively, here is my take:
I keep 9 around ONLY for one thing. I did a side-by-side on Ghost Recon and Jedi II running in both OS's. Hands down 9 is the winner. It wasn't even close. And Falcon 4 (OOOLLLDDD game) was horrific in X. Don't play games?
Toss 9 in the trash and don't look back.