The embedded JPEG in the camera raw is tiny and it's a rendering the camera would have produced IF you set it to JPEG.
The embedded JPEG in the DNG can be very large and more importantly, represents the current rendering you've produced within Lightroom (assuming you invoke the Update DNG). Big difference. I'm referring to extracting that JPEG.
The embedded JPEG in the raw is pretty much worthless.
I think the 5D2/5D3 jpegs are full size jpegs, heavily compressed....but, as you say, pretty useless.
My question to John was whether DNG stored the embedded and the LR update to the DNG?
I asked that because of JohnBeardy's comment about comparing the embedded to the LR output. Though quite possibly he was talking about the jpeg embedded in the native RAW while you were talking about DNG.