I think the DxO measurements are accurate and give you good information about the aspects of the camera they are measuring. There is, of course, many other things to take into consideration than just the sensor when picking a camera.
What I have never been able to make sense of is the DxO overall or case usage scores. These numbers don't seem to make a lot of sense to me. I guess part of the problem is they are only rating the sensor, not the camera.
At ISO 100 the D800 has 2.25 stops better DR than the 5D III. If the 5D III had 6 stops of DR and the D800 had 8.25 stops of DR, this would be a huge deal, but the 5D III has 11 stops of DR and that is more DR than any monitor or print can show. The extra two stops of DR will not be seen unless the dynamic range is compressed using single frame HDR adjustment techniques. In the vast majority of photographs you will never see that extra DR. You can increase the DR of a camera like the 5D III using multi frame HDR, so that extra DR is not a critical factor for me.
Professional and good non-professional photographers have learned to work with light, either find good natural light, supplement natural light or create their own light. I recently heard a talk by an old news photographer. He said when he was sent to cover a parade or similar event, his basic rule was find the good light, then find something interesting to photograph in the good light. There are types of photography, like landscape, where that isn't always possible, and those are the photographers most interested in the high dynamic range in their cameras.
DxO is a company that sells software that among other things does single frame HDR adjustments. They talk about an "exclusive HDR mono-image feature" in their software. They have a vested interest in encouraging camera manufactures to increase the DR of their cameras so that this feature becomes more useful. Without HDR like adjustments in post processing, you will never see the difference in the dynamic range of the 5D III and the D800.