Its more of a cover your arse senario i think, though if you have 4 rubber flexible hoses all swelling slightly under heavy hydraulic load, then fitting only 2 stainless braided ones, means that any swelling thats no longer happening on the new braided hoses, may be increasing on the exisying rubber/rayon braid type.
I have braided on the front(about 2 years ago) and still have not got around to fitting the rear ones, and can report no noticable problems as yet, may ge around to it this summer.