Rain (droplets) shouldn't be much of a problem, unless it's a big long trip from Seattle to Miami
Submerged with water such as a splash is NOT good and/or potentially dangerous. Which is why those fender CAIs are prone to this problem.
Any amount that may replace the entire air stream going into the intakewill equal bad. Rain drops don't do that. Actually, water injection is used in some cars to increase power. So you might feel a power incrase from the result of the rain.
Now the downside is of course the filter would get dirty 3 times as fast.