If by 'in the back of the car' you mean inside a wagon etc then consider the situation when you have a flat and the tyre is covered in mud. Then of course there is the loss of storage room in the back and accessibility... Do you use you vehicle for long trips (where storage space is important) or is this just a w/end 4wding thing?

I've had a flat in wet/muddy/raining conditions and the last thing I wanted to be doing is spending any more time than necessary dealing with said mud covered tyre than just tossing it on the rear tyre carrier (that's already messy enough)...

cheers,
george.