The trouble is he only thinks that ‘some’ elements of the prosecution case didn’t stack up, so would presumably have ended up agreeing with other elements of the prosecution case when under cross-examination.
She has 15 whole life orders. Even if his evidence had cast enough doubt to bring that number down, unless she was cleared of all charges then it wouldn’t make much material difference to her. If they had put him up and he agreed with any elements of the prosecution case for any of the babies then it would have been guaranteed game over for her.
So you can see why the defence might decide to not put him up as without him there could still have been a chance of getting no convictions if the jury had felt that the prosecution case didn’t stack up.