From Benq's site :
"The BenQ exclusive Dual Cooling System(DCS) features Anti-Dust Cooling System(ADCS) and Air Flow Cooling System(AFCS) to effictively reduce the potential for over-heating during high-speed reading and writing. With an integrated heat chimney design, ADCS redirects the heat flow out of the drive and still provides for protection from dust. AFCS accelerates the circulation inside the drive and speeds up the heat elimination through the drive's steel housing."
Where did you see that "Antidust only works when access is restritced. When drive is not in use" and also if there's "no air movement/no venting" which heat flow does the ADCS redirect out of ?
From what I read DCS is ADCS and AFCS working together during HiSpeed writing/reading, or maybe you are right and it's all smoke in the eyes.
But, first of all, the question is "Why at Benq they placed a vent on the drive side if it's useless ?"
Perhaps Eleewhm could give us a proper answer.