A brake engineer told me a while back that around 90% of braking is done by the front brakes. Other than ultimate braking performance (i.e resistance to fade) and marketing, rear disks have quite a few disadvantages: Higher cost (more parts), heavier weight (you have to have both read disks, and drums for parking), more drag.
If rear disks are to be added to a car like the Versa (or any car for that matter), it may be wise to upgrade the front brakes as well to have a balanced system as rstolz implied. If not, the ABS system may be working overtime to prevent rear wheel lock up.