While not sucking up the fumes of the guy ahead is likely the most common reason, its not the most important for not tailgating. I don't want to be number 30 or so at KORD on a snowy day and be butts to nuts on the guy ahead only to have him shear the deice/anti ice fluid from the wing with his engines rather than on T/O roll.
Air Florida accident comes to mind as another reason not to be excessively close.