Randle may be able to take bigger bites than Murray but I still don't think he can eat as much as Murray. It is odd that the coaches didn't use Randle more, but there may have been some unknown issues we don't know about.
Murray was used almost exclusively because the Cowboys always figured he'd be gone after the 2014 season.
They basically ran him into the ground since there was no need to spare or pace him for future seasons on their dime.
As he was the more experienced and complete back it made sense, particularly since Romo was one hit away from a wheelchair.
So go for broke with Murray in 2014 and move on after the season. They simply can't resign Dez, improve the defense, and lock up Murray long term while surviving the cap. Murray was the odd man (or more specifically, position) out and was considered the most expendable and replaceable, given the dominance of the Oline.
This is clear to me that was and is the Cowboys philosophy. They saw enough of Randle in limited carries to prep him for this season and groom him to be a more complete back. It is also why they stashed away Ryan Williams.
Murray was never in the plans beyond 2014 and they made a tough but wise decision. That rankles many but it is the truth.