Just got asked that by another forum member as well.. here is what I told him:
"Applications very similar to these have been well tested in the rain, so unless you're driving through a hurricane / freakish storm.. you'll be just fine

"
So as stated, behind grille applications like these have been used for years now on BMW's (a platform we happen to specialize in) and we have never seen issues come up. With test after test being performed on them and the intakes passing every time, there is nothing to be concerned about in normal conditions. As mentioned though, it probably isn't the smartest idea to drive through an insane storm with these on, or any other open element filter intakes, for that matter.
Shah @ SSR Performance