IMO the author wanted to make Jamie not look like a jerk. It is easier to accept Jamie leaving his wife if the wife is a total jerk, the marriage is unhappy and they are separated, the children aren't his, and it's Leery, who we already hate. We are on board for his leaving her due to that.
Reminds me of Poldark, where Drake was going to marry Rosina, he was up front with her that he'd always love Morwenna, and when Morwenna's husband suddenly died right before Drake and Rosina's wedding, Rosina was all for his going to her and gave up on him right there. Thus the author propped Drake