Wil, I carry one of the Susquehanna kits under the passenger seat of the BPOS. It's come in handy - but then again, that rig sees only hard core trail use. I've rolled the truck with no spillage, but I've extended all the vents and made traps to help prevent fluid loss. The one time I did use it was when a buddy rolled a rock up into the oil pan - we captured the majority of the oil in garbage bags, and shoveled the comtaminated sand/soil int the same bag for disposal later.
Note: Proper disposal raises a whole other issue - it's almost impossible to find a site without paying a hefty fee. Technically it can't be placed in regular garbage or taken to the land fill. I was lucky - our shop was moving at the time and had to pay for a cleanup at the old building location, so I added my junk to the pile for incineration.
I don't think is neccessary for the average wheeler to carry a spill kit. If you (or anyone else) feels you're wheeling hard enough to be concerned about rolling, by all means get one...
The primary problem a "normal" wheeler would see is if they peel a rear diff cover or lose a drain plug - and unfortunately those usually result in a long string if oil being spilled - tough to deal with...but both are preventable.
The typical stamped steel removable diff cover like a D44 uses can be strengthened with a "rock ring" to avoid being peeled open. Hex type drain plugs like those found on Toyota's should be changed to a round low profile style that uses an Allen head socket to avoid having them knocked loose.
Carry a few extra garbage bags - they make a good catch basin or clean work area if you need to deal with oils/greasy items.
Mark