The MD 80 series and I think DC9s had a ventral airstairs which were used for boarding disembarking where no jetways . So in the same way that normal aircraft doors have slides some of the MD 80 series had to have a slide there since it was classed as an exit . Probably a litl more complicated than the usual door slide arrangement but there is no other way out of the looonnngg MD 80 series. And indeed the cabin crew had to jettison the tail cone as part of the procedure. If you look at pictures of the aircraft you will see thesre is a pair of doors at the front behind flight deck/galler, double over wing exits and thats it so the tail slide was essential.