Any bench player should be prepared to play 80mins, that was always my issue with Skelton, he wasn't an 80min player at test level
Of course they should be able to play 80 minutes because they may be required to.
It's a valid part of the selection decision though when considering what their role will be. Different players have different abilities in terms of being able to have an impact off the bench.
A strength of Carter's game is that he can provide a pretty consistent effort across 80 minutes but with that, he probably isn't as good at providing impact across 20 or 30 minutes as someone like Tui might be able to provide (yes, I realise Tui is injured).
The bench has to be able to deal with injuries but a substantial part of the selection decision will be about who can best fill the role they are intended to play which will be to provide impact over a shorter period of time.
I agree that selecting Skelton for that bench role created a risk because if he was required to play 80 minutes he would be found wanting. Selecting the player that can play for 80 minutes but struggles to provide much impact in a shorter period of time is a safer option but has less upside.