There's probably a bit of that, but there's also the fact that Rubezic offers something different to the other two, in that he's aggressive and strong in the air*. MacDonald is very similar to the other two in that he'll be comfortable sitting and watching the play in front of him and mop up behind. There's a lot of value in having the mix of both (McLeish and Miller). In theory, Rubezic should be slightly advanced of the other two, and be the one aggressively attacking the ball, closing the gap between midfield and defence. That means that he should be allowed more mistakes than the other two as he's the proactive one. It also means Jensen and Gartenmann should be constantly on their toes for slip-ups. It's a little bit easier to make no mistakes when you're not throwing your body in the mix nearly as often, so Rubezic probably deserves a little slack at times. Gartenmann is clearly a class player, but I'm not as convinced by Jensen. He's not a poor player, but he's a downgrade on Scales. Last season, Robson could play with just MacDonald sweeping behind two players playing aggressively in Scales and Pollock, whereas we're a bit lopsided this season with a distinct lack of aggression on that left (which is probably why McKenzie adding that at left back has returned our best performances). That's why I think, for now, it's still best to persist with Rubezic and have MacDonald as cover for all three roles.
*Whilst being great in the air, he does seem to have got under the ball a lot this season, which is fairly basic for a guy like him.