The Lyon situation kind of reminds of the Giles situation with England. Obviously Lyon is a better bowler, Giles a better bat etc.
There's a real lack of obvious spin options and the team already has 3 decent seamers plus other reasonable pace options. From a statistical point of view Lyon is an okay option, but really he has simply cleaned up a lot of tailenders and typically ordinary finger spinners tend to get quite good figures in their first 50 or so wickets then things go downhill. He doesn't really have a role in the Australian side. Australia need a spinner who can bowl in friendly conditions or force victories, Lyon is ****e when the onus is on him. Which means he's not really worth much to the team.