Get your point but Mata is not the best defensively and playing him out wide would just leave Cole/Bertrand exposed, which happened a lot of under AVB where Cole was often caught 2-on-1. Adding to that, playing Mata out wide would possibly make him see less of the ball and more crucially, he doesn't have the pace and skill to beat his full back to offer us width in the wide areas as he will definitely come inside more often than not. Mata may not be in his scintillating best when being played as a CAM under RDM due to tiredness(as you pointed out) most likely, but at least it allowed him to get into the game more, see more of the ball and orchestrate attacks and chances for us from there.