Hip belts on daypacks

Do people find they make much of a difference? I've been wondering about replacing my dayback. Today I was in a Macpac shop and ended up looking at their Rapaki line. Most of them have padded harnesses and quite wide & grunty hip belts. But then they have their Vented 30 model, which has one of those arch harneses (with the air ventilation gimic behind it), yet for some reason the hip belt on it is really flimsy, inconsistent with everything else in that Rapaki range and despite it being the largest and likely to be used for most weight. eg. http://www.macpac.co.nz/packs/packs-day-packs/rapaki28.html compared with http://www.macpac.co.nz/packs/packs-day-packs/rapaki-vented-30.html . Click the back view underneath the picture to see the harness & hip belt. I asked a shop assistant and she told me that the ventilted harness somehow does more work to the point that there's less need for a grunty hip-belt. Is this an explanation which adds up for others here, or is it more likely that the assistant was just making something up to explain it away? I've had a string of ventilated harness daypacks (not Macpac ones and not specifically for the ventilation aspect). My personal impression has been that even without too much in it, but especially when I load it up with stuff, I find it more comfy with the hip belt done up and taking most of the weight.