Yes, really it would be best if Rafa was only playing a'dead' rubber , preferably doubles. But there is no guarantee of that and if they put off playing him, his appearance could end up being a crucial one. Especially if Carlos is not 100%. yet.
It's a tricky call for Ferrer to make. They want to win- but it's also been built up as Rafa's swansong- but I'm sure Rafa won't want that to overshadow the team's chances of winning.