You might think that jointing a chicken before roasting it would, at least in one respect, be preferable to roasting a whole one. You can add the breast portions later, giving them enough time to cook but not enough to dry out. But my experience suggests that roasting the whole bird works better.
I was cooking just for myself one Saturday. I removed a leg from a chicken, and browned it gently in oil and butter while my roast potatoes were getting a 30-minute burst of gas mark 6/200 C heat. Then I lowered the setting to gas mark 4/180 C, turned the potatoes, and placed the chicken on top. I squeezed on some lemon juice. I gave the dish another half an hour. It was fine; but the chicken was not as tender as I would have liked.
The next day, for the whole family, I browned the rest of the chicken pieces, and put the remaining leg (cut into two) with the wings into the oven at the same time as the potatoes. I felt that I could risk an oven setting of gas mark 5/190 C, to brown the potatoes without drying out the meat. Again, I turned the potatoes after half an hour; I added the breast pieces with 20 minutes to go.
The chicken -- even the breast meat -- was not as succulent as it can be. Perhaps a whole bird retains more juices.