Personally, I tend to avoid partitions on secondary drives, but I do keep one for my OS installation. For example, right now I have a 160GB boot drive that has two partitions; a 30GB one for OSX, and the rest for data. My data drives are all single partition.
My reasons for doing this are:
1) I like having my OS on its own partition, so if I need to I can wipe and reinstall or reinstall from a backup without affecting the rest of my data. Very convienent in case of emergency. Although I haven't recently seen any OS problems that can wipe an entire partition, this also protects the rest of the data if that happened for some reason.
2) Backups are easier; since the OS partition only has OSX and my "most important" data on it (in the various users' folders), I can keep a like-sized partition on an external backup drive and use CCC to clone the whole thing to that. This saves me having to selectively not back up non-vital data that is on the same physical drive, but not the same partition (most of the data partition is video scratch stuff or junk), and it gives me an easy-to-boot-from backup.
3) Permissions. If I've got 20 folders of data that I want to share between users, and only the one partition, I have to put that data at the root level of the drive, probably in a subfolder, and set the permissions on that folder accordingly. By having it on a seperate partition, it's easily accessable to other users without permissions fiddling, and I keep the root level of the OS partition clean.
4) If I've got a program that's dumping vast amounts of video to the drive, I don't have to worry about eating up my entire scratch space, since I'm saving to a seperate volume; the OS keeps enough space for page files even if my data drive accidentally fills up completely (which has happened several times). This is slightly less of a problem under Panther, but it also just encourages me to keep some free space on my OS partition since I'm not tempted to chew up every last bit of space on that partition, which is a good thing.
All in all, I just find it convienent to keep an OS partition, but I wouldn't do it to any other drives unless, like MacBandit said, they're a backup of your boot partition.