Yep, I think you've summed up things pretty fairly and so it does come down to personal preferences as to what you end up riding. For some, It's the very basic nature of bike technology that appeals, after the over sophistication of modern cars. And just as it's hard to see why people when faced with similar cars with pretty well the same specs will choose one over another (Honda CRV, Toyota RAV4, Mitsubishi Overlander), I guess it's very much the same with bikes/scooters. It's all in the eye of the beholder.
Having said that, what you said about "fanging" it through the mountains on your 650 being so different to the same experience on your Vespa, I think the Honda SH300 points a little towards the ride you would get on a Scarabeo 500ie. Look back through these archives at the Harley biker who rode a Beo to Philip Isle last year and compare that with Allen's trip, also on a Beo. Those machines allegedly hill climb easily at the 160+kph mark.
Just looking at the last Scooter Mag, the move to larger wheels, by scooter manufacturers, is significant but not unsurprising, given the roads here in Oz. On the other hand, while we were at this year's National Scooter Rally, we camped near a group from the UK. They had traditional, manual scooters with all the Mod paraphernalia and you could see the love that had gone into their restoration and primping. Ours were two Scarabeos, a 250ie and a 500ie. Although we got on well and shared many hours yarning about common interests, I think one of their statements is quite telling: "I can see they're lovely machines, but I can't see them as scooters."
These traditional scooter owner/riders don't see big wheels as part of the scooter essence. So they would say I'm riding an automatic bike. I find it hard not to look at the classic shape of the Beo and not see it as a scooter, but then my eyes are definitely bias 
I certainly agree with you on the point of "off road" stability though. I keep our little river cruiser about 20km from our place, where the last 5km is a pretty rough gravel track. I've taken both the 200cc and 500ie Beo there and neither has liked the conditions much, especially traveling in the left hand track with a slope towards the left, near the cliff edge. The alternative is not better chancing meeting a car on blind corners while stuck out in the right hand track. Traction is only possible holding the scooter upright at speeds of between 40 - 55kph. The few times I have tried to lean on corners have almost resulted in a slide and drop. With different tyres it may be better, but as I do most of my riding on roads and highways, offroad tyres would not be a good idea.
I really do like the ride of my scooter, the fact I get 26km/ltr normal use and 30km/ltr on long runs, at 110kph. These figures are a lot better than some 250cc scooters and definitely better than the 20km/ltr of most 650cc bikes. Sure, the Beo is really only 460cc not 500cc, but the big class, big performance, big style, big economy make a definite big appeal (to me at least).
ciao, Mike
Riding big red "Audrey" an Aprilia Scarabeo 500ie, and "Silver Pixie" an 08 Vespa PX 200,
previously an Aprilia Scarabeo 200, and originally a Suzuki F50 Step-through