Cmac... Youngs is a poor runner?! That's the main reason youngs was picked for this tour! For his ability to exploit gaps and beat defenders.
The aspects of his game I'd question are his decision making, and the quality of his kicking. And to be clear, I'm not saying youngs should start, but that his ability to inject pace off the bench is why he has to be there. I'm actually a fan of Murray, but when the three scrum halves for the tour were announced it was immediately clear that Murray was going to have to do something special to get into the test squad, since he's not entirely dissimilar to Philips, who gatland will always prefer. And I think so far Murrays game has been a bit messy, with mistakes creeping in. Additionally, whilst Philips arguably should start, it's youngs who has played the toughest conditions/games so far, and in that sense he's perhaps the best prepared.