The argument would be that the Senate Rs will call Biden to testify. He's already said he won't appear. But part of the case against Trump is that he is obstructing by refusing to let people appear, and not appearing himself.
So, on the one hand that could undermine the case against Trump and, on the other, it will be used against Biden if he's the nominee.
So, on the one hand that could undermine the case against Trump and, on the other, it will be used against Biden if he's the nominee.