default, because i don`t know how it works ;)
Lower PT samples mean more sampling will be devoted to AA, and aside from clearing edges, AA also clears DoF and motion blur, so the more significant portion of your images is blurred by DoF/Motion blur, the less GI samples you should use to devote more processing power into these effects.
On the other side, in situation where you don't use any of these effects, or use them in very subtle way, and the situation demands lot of GI sampling (rooms with small entrances where light comes through), you might actually want to increase PT samples.
Think of it as a priority defining value. Smaller number means AA+DoF+MB has priority, larger number means GI, light bouncing and refractions have priority and value of 16 being a good starting point.
In such a blurry scene like you render here, try 2-4 PT samples... ;)
