i understand the point that you are making. remove whatever is unecessary or appears to be redundant as a weight savings measure.
from your post, i gather that you don't feel this way. it seems that you're just saying someone else might glance quickly at a two-buckle design and dismiss it as nothing but redundancy.
in this case, i believe that it is only an appearance of redundancy. a purpose is actually served by the twin buckles...
if anyone hasn't seen Dan McHale's fine packs (arguably the best made packs in the world from what i gather from reading the comments of true experts who own his packs), check them out at McHale Packs
i'm guessing here and am probably missing the most important aspects of the two-buckle design, they seem to provide a means of differentially tightening the belt's top vs. the belt's bottom. this would seem to provide, at the very least, a more secure "connection" of the belt with the wearer's pelvis, and more comfort. for the somewhat heavier (sometimes non-L/UL) loads that these packs are designed to carry. this would seem to be a very good idea.
i believe, i could be mistaken, i'm guessing here, that some mainstream non-L/UL pack Mfr's/designers, whose packs i've seen, have tried to copy Mr. McHale's idea by using two "runners" of lesser width grosgrain sewn on top of the main belt. these runners run back along the top of the main webbing from an area near the hip-belt's main large single buckle to their own additonal adjusting buckles located on the padded portion of the hip-belt. these tension adjustment buckles (they do NOT hold the hip-belt on the wearer) provide only a "poorer" means (as cp. to Dan's idea) of differentially adjusting the belt's tension. some other pack's i've seen sew at angles, starting much further away from the main buckle than the first method, shorter grosgrain to the main hip-belt webbing. these too return to tension adjusting buckles on the padded portion of the hip-belt. don't misunderstand my poor description, what i'm speaking of here are NOT the common forward pulling adjustments, generally found further towards the rear of the hip-belt, that pull the pack bag bottom towards the wearer's back - these "adjusters" are found on many mainstream non-L/UL packs. nor, am i speaking of the "Scherer" cinch design used by some Kelty packs.
to my largely uninformed mind, both of these arrangements seem, for a number of reasons that immediately come to mind, to be inferior to Dan's design.
i could also imagine that a hip-belt could be designed such that with proper (even equal) tensioning of the twin-buckles afixed to top and bottom of the hip-belt, would cause the center of the hip-belt to flex/bulge inward causing it to ride more securely on the iliac crest. not sure that this is how it's intended to function - just making wild guesses here. there has to be (a) good reason(s) for the design, or Mr. McHale would not have used it.
Mr. McHale, if i've misunderstood the intent of your two-buckle design or left out other advantages to it (undoubtedly), feel free to post and set me straight, and educate us about the nuances of your fine design - you'll get no argument from me.
also, one might argue that two smaller buckles and their associated webbing, actually weighs less than one larger buckle with wider webbing. while this, to my mind, is insignificant for the McHale packs, given their non-UL design parameters/goals and greater empty pack weight, it might be used together with a a number of other weight savings measures for a L/UL 15oz pack like BMW is bringing out. i've read that, in the L/UL world, webbing is one of the heavier materials used.