Our three Macs are “sliced” differently, for different reasons.
My AluminumPB (for travel – and on which this in being composed) has only two partitions of its 75 GB (mol) drive. One (15 GB) contains a spare OS in case something totally unexpected befalls the “”main” OS – on a 60 GB partition.
A "safety net" for traveling far from home better than anything else I’ve been able to think of.
Before 10.4.x came along, I maintained a separate partition for Documents on it, but when my home Mac (a iMacIntel) “joined” the family - and Classic went the way of the Dodo Bird – I decided not to partition it, just to see why Cupertino designed OS X they way they did. I then found it very convenient to have documents default to an established folder with many, many sub-folders for retention and storage.
Lillian’s G-5 iMac has a multi-partition scheme (OS/Apps/Utilities + Docs + spare OS + Archive) Since it’s a G-5, it continues to have Classic operating (in the “open) to run her favored WordPerfect (albeit somewhat shakily). She became so accustomed to having Documents stored separately from pre-OSX days, that’s she’s more comfortable doing so today. Surprise, surprise.
Each these Macs (and their partitions) are regularly backed up to similarly separate partitions on a 500 GB external FW/USB2 drive.
Stashed elsewhere in our house is (our not yet quite outcast) old Pismo with an early OSX + a separate "retail" OS9 on differing partitions, and docs files going back 15+ years, that I may wish to access. It has an airport PCMCIA card, so I can print those files should that be necessary.