I don't think it is complicated. They don't exactly know what the have with Seigler. He's a lefty, Durbin struggles against righties...thus, you play Seigler to see what the kid can do while also playing the numbers. If he stinks, he goes the way of Dunn. If he is good, you have a dynamic platoon with Durbin.
After everything that has happened this year I find it odd that people STILL write off unknown players before even giving them a chance (a "chance" is more than 3-5 games).