well your an architect... so your close enough imo. the two disciplines overlapped a great deal at my last company. we had one or two on staff. i'm sure you've already guessed it's for strength and uniform stiffness.
i'll try to recount my previous explanation before the boards crashed... if you can, look at the swingarm on a stock mille. you'll notice that it's asymmetrical. it's got triangular bracing on the left (chain side) and the right has the "banana shaped" style that a gp swingarm would have to give clearance for the large expansion chamber "tuned" pipes that "2-smokes" have. while maybe lighter due to the lack of reinforcing triangulation, the trade off is strength and stiffness on that right side.
while not a 2-stroke of course... the stock mille (unlike most other twins ie. rc51, 996, tl) utilises a single system exiting on the right. being that it's a subsitituting a single exit pipe for the usual 2, the diameter has to be significantly larger. so it seems, the design team in noale figured it would be cool to add both function and a "gp-esque" styling touch to the mille's design. after all, heretofore, aprilia's racing heritage had been gp's.
well they were right, it is cool...!!! that stock swinger is a sweet piece o' kit. as far as i know, it was the first to weld together both castings and extrusions for a production bike. it's relatively labor intensive compared to just welding together extrusions as is usually done. it's primarily reserved for frames. yamaha's did it with striking effect on the original R1 frame. then aprilia did it for both the frame and the swingarm for the mille. and now honda's using it for both the new swingarms on the 954 and the rc-51.
sadly, while "good enough for government work"... the use of it's asymmetrical design in the heavy weight 4-stroke class of WSBK, was letting troy corser down. the fact that they've taken to changing it, seems indicative it was clearly contributing to the "mysterious" tire problems corser complained of from the very first round last year...??? i guess the figured, hey... all the other manufacturers (ie. kawi, suzi, honda & ducati) substitute "works" swingarms... why not us...? it seems that during the off season ('00-'01), whitveen, had found the much needed extra power they were looking for, but under "race conditions" it was being revealed that the stock swinger hadn't kept up with the pace of developments. me thinks they were just SOOOOO focused on increasing the power, they overlooked the need to develop the chassis along with it...? needless to say... that was a CRITICAL engineering oversite...
and the fact that they seemed to be ignoring corser's contentions that something was wrong...? only made things worse. it seems so F'n shortsided, that after you were SOOOO trusting of his obvious talent and willing to develop the bike around him based on his input... why would you suddenly, NOT LISTEN TO HIM NOW...!?!?!?!? somebody should've gotten FIRED, but it wasn't supposed to have been corser. i see this swingarm as a vindication of all his complaints voiced from the very FIRST round of valencia last year. corser's clearly a better man than me, 'cause i'd be pointin' fingers and shoutin' I TOLD YOU SO...!!! from the hill tops. LOL
after all, in the quest for power, whitveen had already long "chucked" the single exhaust and adopted a dual system. they obviouly weren't experiencing clearance problems on the chain side, so the "cut out" provided by the "boomerang" on the right side was now redundant and prioritized "form" over "function". as we all know, it should be the other way 'round.
so it seems they've taken some of the faceted "hav-blue" skunk works design elements from the swinger of the "RS-cube" and built themselves (and haga) something better.
