What Pac Ace said.
I use the enbedded jpegs (shooting RAW with medium jpeg) as proof images and to give a copy to models at the end of the photo shoot.
It takes mere seconds to extract the jpegs from the RAW files, where as it takes forever to convert the RAW files into something useful relative to the expectations of the models. in otherwords the embedded jpegs use the auto white balance, contrast and sharpenning commands of the camera so the image is close to ready to go, but the RAW is raw and needs to be post processed in order to make it look nice. There is also the fact that to convert the RAW images to jpeg takes about 3-4 seconds per image on my computer, meanwhile extracting the embedded jpegs goes at about 15-30 images per second. Huge difference when taking over 500 images a session.
Also because PS does not have a good image slide show capability, which I use for seeing which images I want to spend effort on, I use a different program and need jpegs for that, so might as well embedd them right off the bat.
Finally using medium jpegs mean I can usually get all of the session on on e CDR to give to the model, so while she is changing I am doing my final download, extract and writing the CD and it is ready to give her within 5-10 minutes of the end of the session versus when using RAW only it took about 45-90 minutes to produce a CD copy of the session.
(note the jpeg is in the file, but is not part of the RAW image itself, which is also embedded in the RAW file as RGB channels - If I understand the file format correctly)
Thus why I personally use embedded jpeg version of RAW. Of course I have have enough CF card capacity and eith my computer or picture pad that it would take a lot of effort to run out of memory space.