I put my older son in a booster seat back there from time to time. It fits nicely, and he really enjoys it.
But to me, a baby seat is a bit different. While it does physically FIT, there is no LATCH system in the rear two seats. [Nor, annoyingly, is there a proper one in the middle seat of the 2d row, I might point out.] So the baby seat is not as tightly fixed as I would like. Further, it is a bit tricky to buckle/unbuckle a baby in a car seat back there.
My wife has actually forbidden the practice because she can't see the baby very well back there; nor does she feel that she quickly could get to the baby in an emergency.
Just my thoughts.