Haven't noticed it being an issue - no surface scum on my 3g. Mind you little skimmate of late either (might be time for a cleaning).
But think about your concern for a second, these spill water down onto the surface of the water, mixing the scum down into the water (where it can be picked up by the intake) It would likely be different if it was setup like a cannister filter where intake/outlets are both underwater.
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