OP here, just want to add a few things and respond a bit.
First, I almost always use a tripod and I'm well aware that an ISO of 100 is the ideal and I strive to keep the ISO as low as possible. However, there will always be some circumstances that don't allow for ideal settings. Sometimes even with a tripod you just can't get the shot at ISO 100, particularly when you're stopped down to f/8 and using filters, which is where having the option to boost ISO becomes a big help and may make all the difference.
Second, of course the 5D3 is not the perfect landscape camera. There's really no such thing. It won't suit every landscape photographer's needs. The 5D3 will suit the needs of some and the D800 will suit the needs of others.
So I think it's a mistake to over-simplify the 5D3/D800 debate to merely "which one is better?" I wouldn't argue that one is inherently better than the other (at least not yet anyway), but that they're arguably equally as good but optimized for different conditions. The D800 seems optimized for more traditional, by-the-book landscape shooters shooting in ideal conditions, and if that's what you want then I'd completely agree that the D800 would be the better camera for you personally. The 5D3 seems optimized for shooters who want more versatility and shoot a bit more in less than ideal conditions. Being an avid hiker and cross-continental cyclist, I appreciate the versatility, so it seems to suit my needs better. Just because the 5D3 doesn't suit your landscape needs doesn't mean that it doesn't suit any landscape needs.
Besides, just because the 5D3 wasn't the high MP monster some were hoping for doesn't mean that such a monster isn't a possibility. I think it's very likely that Canon will put out a high MP camera.