well, many remove it from under even without adding an aux tank, cuz the spare under will
- reduce ground clearance
- get shredded on sharp rocks
- may be hard to take out altogether when you need to if you are in difficult terrain
- may be a pain to remove even in the driveway -given the hokey lift mechanism on the 80 at least- compared to one located in the back, for ease of rotation for example
- bit harder to keep properly inflated, much harder if it's upside down

- be limited in size as far as fit under there in some cases
- display less safari-style effect if you are trying to snag some exped-inclined gals.. (Warning: the Sierra club gals only go for Subaru-types... and who cares about those hippies anyway?)

- more no doubt