It's not about whether or not they could be useful. Akatsuki is only fighting the villages because they are trying to get to the two Jinchuuriki. Once they get their hands on them, They've won. So putting the two in a position where they could be taken is a foolish mistake. This isn't like the Nagato invasion where there was no one else that could fight on his level. We now have the combine forces of all five major villages, Meaning there are plenty of ninjas on the level of those two that can fight in their place. So there's no need or reason to put them at risk.