If Rettig was as good as some people seem to think he is, he would have beaten Laviano. The coaches want to win more than any fan. Even if you don't trust Flood, Friedgen and McDaniels certainly were a big part of the decision. So if you are inclined to think Rettig was better, the most you can reasonably take away from this is that they were nearly even and the coaches chose the other guy.
I can't see how any fan can second guess the staff in that circumstance, or how anyone would have a basis to claim the other guy should have been chosen. If they are so close it is a coin-flip, why not just support the decision made by the professional coaches?
For me, if it is true that they can run more plays with Laviano then Rettig, I would think that is a huge factor. The decision also fits the standard way of handling these things across college football, which is that in a toss up situation, whoever was with the program longer gets the nod.