As far as the extent of this tournament goes I can't disagree with you, but it's an overgeneralising blanket statement to say that these players have nothing in their arsenal except straight runs.
Why? Have you seen Jon Davies on form? Surely not a Shane Williams, or even Henson, but he can occasionally ghost through gaps in the line and he has great pace. I feel pretty similar about North on good form too and even though your point has a little more basis for Roberts and Cuthbert, they both have a little more to offer than how Wales has been using them as of late.
Yes, similarly to Roberts he is a good player when used effectively, and is good in defence. However he is a glorified forward in the backs, who can't pass and has unacceptable passing skills for an international centre, is a serial two on one botcher, his combination with Roberts doesn't gel, and yes his main weapon is to run straight.
As for North. When he first came onto the scene, he had a tremendous sidestep. However unfortunately he seems now only capable of running route one at the moment. A shame, as there was a point where I thought he could become a World XV winger.
It's not necessarily a criticism of all these players that they can just run straight by the way. But to do so effectively, we need someone who can give them space and give them ball to run onto at pace and target gaps. Otherwise it is just too predictable, as we saw Argentina, Samoa, New Zealand and Australia in November weren't too troubled from the Welsh backs. Wales only scored 3 tries, one was a 13 man maul, another an intercept, so we scored one constructed try in 4 matches in a match that was already long lost. Not good enough.
And below is the man who can do this. Watch this GIF, and replace MacKenzie with North/Cuthbert/Davies/Roberts running straight at pace into a hole in the defence. Now that would be using them running at pace straight effectively. Under the current gameplan, the ball hardly ever gets past the centres. The lack of passing is negating the threat of two dangerous finishers on the wing, it would be the same if Eli Walker were on the wing as well.