I suspect the thought of the tail blocking out half the balls and slowing the rate any lead might accumulate didn't appeal to Amla. Had Morris still been there he might of been tempted to bat on into the morning and leave England 50-100 behind with 5 hours to bat.
On the plus side England only need to bat 2 sessions to kill the game on what appears to still be a good deck. Key for England is tempo, score at 3.5-4 runs an over and get 150+ ahead by lunch without too many down. The game will be virtually out of reach. If they crawl to 100 by early afternoon then SA will still have a chance even by tea.