The System has done a bit of both.
End of the day if a guy is playing as well as Murray last year you ride him til the wheels fall off.
If you switch backs in and out a lot of times it signals what play is coming because the entire offense isn't open to each guy.
Dallas with Emmitt and last year under Garrett/Linehan was pretty simple and maintained one back that ran everything.
When Murray came out his back ups had smaller playbooks. Because they played so little it would be hard for teams to key on those or gain advantages.
It we did rotate it all year it would have been possible to key the plays of each.