well i dotn know for sure so dont quote me on this but based off of physics this is how i see it robbing power...
it is a section that allows water inside the intake to pass through and out through that white material so it doesnt get sucked into your engine.
Last time I remember SCC did a test on filters almost all of the name brand filters needed to be practically submerged fully in water for them to suck up enough water to cause damage.
I also assume because of the design of that bypass valve the ater runs along the inside of the pipe along the walls.
Based on that anything that can allow water to pass through it and out of the intake will allow air to pass through it also, which results in a loss of "pressure" or amount of air the intake is actually bringing into the engine. Since most people dont drive in foot to foot and half deep water they really should have a use for the valve 99% of the time. the v shape of the valve is what catches the water but since the engine is pulling air in so hard I would also think warm air can go in that valve as easily as water out, defeating the purpose of a CAI.
so yes it can techincally rob power, and I have only seen it used with actuall race design intakes where the filter is practically on the ground so it sucks up the most air, then again, in most races for imports it isnt raining, or hard enough to worry about that.
If your a paranoid person aboutt his stuff and you have taken the bootm plastic piece off the fender below the filter than you might want it.
I dont.