Appointing a Knesset speaker would give the opposition control over parliament for a while, as well as the ability to pass a limited amount of legislation. But the price could be dramatic: new elections under difficult circumstances, plus a certain political cost for parliamentary cooperation with the Joint List.
Not to mention the small matter of who the speaker would be. Elazar Stern is not exactly a popular figure with Lapid and Lieberman, to put it generously.