Great thread! Well on its way to providing a very complete set of information about the Oregon including weights, dimensions and a load of opinions!
In a word, floatation. I seem to remember the same question being asked back when Bruce Bundy and Dave Poznanter started using 3" tires. The answer then was ‘flotation, flotation flotation.’ 
The downside is big bounce. I imagine some playing around with tire pressure can make it better suited to whatever trail you happen to be on, but too much pressure makes it ride like a rotating trampoline. Though even that can be fun.
I got to ride it with Josh on the trail at the Vancouver MUni Weekend, for a mile or more. It was really fun the way that thing eats terrain. Like the step-up from smaller tires to 3" downhill tires that many of us did about 10 years ago, it makes nearly any given terrain easier to ride on.
I think the idea was to spec it simply and let people upgrade, either when ordering or later on. This keeps the initial price from being even higher.
Uh oh, somebody talk MuniOrBust out of buying one of these before it’s too late!
But just because it’s not for you doesn’t mean it’s not for someone else. Even posers. The “Hummer bar” has just been raised. Some will buy it because it has the biggest tire, and for no other reason. No worries, every sale helps support the development and improvement of future models and designs!
All this talk of weight, while helpful to the overall thread, is kind of missing the point. If you’re after light, you won’t be riding this. However non-heavy it may be, the rotating mass is not going to be minimal if it’s to be a strong wheel. It’s going to tend to feel heavier than a skinnier wheel of similar diameter. The question then is, is it worth it to you for the ride the massive floatation brings?
Using that thing for XC would be a waste. Unless maybe if it’s really bumpy XC. The purpose of that wheel is to eat bumps. I bet it’s really great on rough, rocky descents (with the pressure dialed, that is).
Here’s a picture of the prototype we got to try at MUni Weekend last year.
