I stood for hours on a beach in the rain and wind-blown sand (the edges of Hurricane Sandy as it started to impact New England), with my 1D X and 600/4L IS II. I'd not have done that with a 7D or 5DIII.
Just to put this out there:
I've stood on a sandbar a short slosh from a snow- and ice-covered shore on a lake, in driving rain, sleet, snow, and hail, on a freezing cold day with 60mph winds and through at least one downburst (where winds can reach as high as 150mph) last March 2012, with my camera and lens completely drenched while photographing birds, for about 6 hours at one of the state parks and wildlife reserves here in Colorado. The camera? A 7D and 100-400mm L lens. That lens is NOT weather sealed...and the camera didn't skip a beat, all through 2012. I've used it in heavy snow and rain on many occasions.
(Canon 7D w/ 100-400mm lens in driving sleet with 60mph gusts; later a downburst almost knocked me off my feet around the same area.)
My 7Ds first real use was up at around 11,000 feet in Rocky Mountain National Park, sitting on a frozen lake (Dream Lake, above Bear Lake to be exact), where 40-60mph gusts were blasting me in air that was already well below freezing. The camera was pointed directly into the wind in order to get the shot. I was using the EF 16-35mm f/2.8 L II lens at the time, which is a weather sealed lens. The camera body was ice cold to the touch. Again...didn't skip a beat. I'd only had the camera a couple weeks at that point.
(Canon 7D w/ 16-35mm f/2.8 L II up on Dream Lake, above Bear Lake, below Hallette Peak and Flattop Mountain on the Continental divide in Rocky Mountain National Park. Winds were constant, at least 40mph, with significant gusts of at least 60mph, if not more. I remember having to melt little divots in the ice to keep the tripod in place, and remember on the slick black ice being blown backwards while standing still. The arctic storm blowing in, which later dropped as much as 10 feet of new snow, can be seen just behind the peaks in the space between them.)
I think the quality of weather sealing on the likes of the 7D and 5D III is underrated. The real contrast comes when you see guys tromping and even crawling through muddy trails, swamps, and other extremely and continually wet regions in the rainforests for WEEKS on end, and the camera is usually soaking wet and covered in mud and other grime on a continual basis....THAT is where the 1D X really outshines the alternatives. Rain driven by wind a couple hundred miles per hour? Really cold weather? I wouldn't really call those significant differentiating factors. Anyone who has heavily used a Canon 7D would know it can handle that kind of weather quite well.
For all the rain, mud, and grime I've gotten on my camera, the only thing that has ever caused me a problem was a bump or drop recently. Not exactly sure when it happened, but I apparently banged the 100-400mm lens (while it was attached to the 7D) within the last couple of months. That seems to have knocked one or two of the elements slightly out of alignment, causing me some hassles. I dropped the 7D a few feet into gritty, grimy, muddy sand late last year. Scraped up the top LCD a little in one of the corners, but outside of that, after cleaning it off, it still generally looks brand new, and functions just as the day I bought it (well, better actually, as it's had a couple of firmware upgrades.)
Canon's pro-grade (xD) bodies are very rugged. The 7D, and I presume the 5D III as it's a higher line, can take a hell of a beating and keep on clicking away. Personally, having had first-hand experience with it, I would be far more worried about the lenses. They are considerably more fragile devices, even L-series lenses. I'd be willing to bet a top-end telephoto prime could take a bit of a beating...but I would be careful with the 70-200/2.8.