I'm always rather impressed by the Sigma UW lenses - the 10-20 f3.5 I had on the Pentax was compact, well made and sharp - cheap too. But basically I do agree - I have similar lenses to go with my A900, and although the telephotos remain exciting, the midrange lenses do become decidedly blah.
The best, smallest and lightest WA lens for the NEX 7 is probably an Olympus E-PL3 with the 12mm. It's tiny, has an articulated LCD, weighs only around 450g with batteries (150g less than the Sigma 10-20 with adapter but without a camera) and serves as a backup camera should you drop the NEX into the river.
The lack of high quality lenses is typical for problems with many Sony products: They are very well executed from so many points of view, and extremely attractive and well designed, although most of them have their little "Sony quirks". But they often fail to see the product in a broader context. Walkman is the typical Sony success; you load a cassette that is a world wide standard, you plug it into your ear, and it works. The Elcaset was the opposite. The product was fantastic, but they forgot to consider the fact that almost everybody on earth who could afford to buy one, already had a music system that worked well enough... and it was at least twice as expensive as the competition... and Sony was the only manufacturer supporting it.
The Alpha mount was a lucky strike, since the product line-up was already more or less there together with a customer base. NEX is a great idea, and partly well executed, but it fails in two major areas: It has a menu system that nobody asked for and that alienate many photographers, and it's seriously lacking on the lens side. Why couldn't they, instead of the Frankenadapter which must have cost some serious work to develop, have gone to Zeiss and asked them to make a WA lens too. I seem to remember that Zeiss has a few WA tricks up their sleeve.
To stay with the cassette terminology: The Frankenadapter reminds me of the old Nakamichi 1000 cassette deck; fantastic quality, but you didn't want to carry one unless you really, really had to. Not that it was meant to be portable to start with, but the Frankenadapter doesn't seem to be very portable either.
One could of course live with the 7 and just the 24 and 50mm lenses. I only had two lenses for my OM-1 the first number of years as well. But with the OM, I knew that it was possible to buy some high quality glass if I wanted. Lots of it. Even the 4/3 system included some rather hot glass from the outset, even if there was just one 5MP body to choose from. The lack of good, compact primes for the NEX, and given Sony's history of giving up if the hill becomes too steep, would make me reluctant to invest in this undoubtedly very fine camera if I had the monies to pay 3,000 shiny silver dollars for that and the two good lenses. Which is a pity, because it seems to be a very nice piece of equipment, more or less tailor made for the kind of photography that I do.