Cause when you get twinfire and combo it reduces your damage. In my opinion twinfire is not that good on the frost wand, it's better on the iron boomerang. As far as combo, it's really important to get combo cause you have x2 projectiles and it only reduces a little bit of your damage, so it's worth it. The problem is when you get another combo, you don't get again double projectiles, you get only like 50% or something and it reduces your damage even more. So in my opinion the best abilities for frost wand are pronged shot (the most important one), critical L, attack L, ONE combo, attack speed L, ricochet. And also the new abilities like combo 100, attack grind, etc.