There is no standard was of defining disk partitions. It is more up to you how you want your partitions.
You can use folder to do the house keeping that you want. I think that most people that use partitions are using it to keep their various sustem software separtate, (i.e. OS X, OS 9 and personal data). One advantage is, (if your system can boot os 9) if the os x partition is corrupted, you can still boot your system or you can boot say panther, or jaguar if they are on a separate partition.
The question a raises, you need to define partitions?
Just one note if your disk were to crash you would loss all of you partitions.
I am currently using a 20Gb disk that has three partitions: OS X which is defined at ~5GB, OS 9 at ~5GB and my data disk which is a little less than 10GB. I purchased iDVD and could not load it due to disk space limitations. I needed 1.2 GB on the OS X partition and I do not have it. So partitioning a disk can hurt you. I am looking at getting Panther. In which I may remove my partitions, to avoid this problem next time. I will not be loading iDVD now since it only will load if you have a superdrive installed, which I do not have