What's the Diffuse Value, exactly?
RGB and Diffuse Intensity in the 0-100 interval are almost the same thing. Try this: add a matte surface and render with 50% intensity and white light. Then render again with 100% intensity but change the light color to midgray (value 127 or 50%). Assuming you are not using linear workflow, you'll get exactly the same render (apart maybe a small rounding error).
Diffuse Intensity is nothing more sophisticated than a multiplication. Nor does it need to be more sophisticated than that.
So How Bright a 100% light is?
It's as bright as you want it to be! In an unbiased render engine, thing would be different. But in most production renderers, light intensities are expressed -or treated internally- simply as absolute values.
The example above demonstrated that you can either adjust the surface's reflectance or the light intensity and get the same result. It does make more sense, however, to keep surface reflectances fixed and vary the light intensities instead. Anyway, the math is the same.
How does this work with Light Probes?
Now, that's an interesting problem. Altrough some light probes are more accurate than others, it's generally up to you to decide how bright 100% is -even with HDR lighting.
Referring to famous probes, as found on Paul Debevec's site, try loading the kitchen probe and measure the left window's intensity. The values reach 80 (8000%) on the blue channel.
If you lower its exposure you'll see that outside the window there is only a blue sky, no direct sun.
Beach probe, being a sunset, tops at 12 in the red channel near the sun. But wait, ennis probe shows a sunset as well, yet its sky is in the 60 range.
So the question holds: how bright is 60 or 80 or 12 in an HDR? Unless you have access to measurements taken while shooting the probe you don't know.
Real world materials have fixed reflectances, however. A mirror is close to 90% (specular) reflection while a wall may be (diffusely) reflecting as low as 20% of the incident light. Black velvet seems to be around 5% diffuse reflection.
Using this kind of values allows to easily setup any scene, as long as we adopt a strict linear workflow with gamma correction.