it seems if it doesn't net a noticeable performance increase it's hard to argue against.
when all people do is look at HP, quarter mile, and 0-100 times they don't really give a crap about that other stuff.
I actually sat down with someone and calculated out all the 'extra' cost of owning a duc. At least on a 2v, it all came down to 12k valve adjustment(opposed to 15-20k for varying i4's), belt changes, and a slightly more frequent chain/sprocket replacement. The unfamiliarity of some systems just freaks people out. I had someone completely against the idea of a hydraulic clutch no matter how much evidence I had to their advantages.
I think the biggest stigma is 1) lack of parts availability both from the dealer, from the aftermarket, and used (ebay, CL, etc) and 2) fewer (or zero, based on your location) independent mechanics who work on them.
Add the additional problems of a perceived lower parts spec on the 'smaller' bikes, even if there is no performance advantage, and you have a pretty easy argument AGAINST buying a duc for someone with a more frugal budget. My contention is that most people don't own a particular bike long enough to really see a considerable difference.