I can understand resting Siddle - he bowled the most overs, and was obviously under strain. Even before that final spell he'd been stretching and wincing. But I think Hilfenhaus should have been a good shout for this. Perhaps he was OK and they just wanted someone fresher and it was a convenient reason.
Lyon would be a good chance to play because: a) Watson is back and can bowl - so a 4th paceman's value is lessened, b) If Starc and Johnson both play, then he'll get their rough AND he'll get the end which takes the ball away in the air - perfect for him, c) I'm not sure they want Clarke to bowl too much and he'd have to bowl a fair bit if Lyon doesn't play and the quicks can't put a hole in SA. Given SA batted a long time in Adelaide, I think it's a fair assumption spin would be required since the chances of 4 quicks blasting them out are fairly low. Then again, we're not entirely sure what the pitch looks like...