There is no difference in IQ for taking photos with camera set to RAW+JPEG or RAW by itself or JPEG by itself. RAW is RAW which ever way the camera is set and the same applies for JPEG.
However, there is a difference, sometimes a huge difference, between RAW and JPEG files.
When taking a RAW photo, the camera produces an image exactly the way it sees it with absolutely no in-camera processing or compression. These files are typically larger compared to its JPEG counterpart. On a high pixel camera like the 7D or 5D MkII, the RAW files will exceed 20mb and for some will approach 30mb.
JPEG images on the other hand, are highly compressed and processed. The camera strips out most colours that the human eye cannot perceive thus creating a much smaller file (compression). Using the above example of the 7D and 5D, JPEG photos average 5-7 mb each, making them much smaller than their RAW counterparts. The photos are also processed by the camera to produce saturated colours, additional sharpness, contrast, etc. This is particularly true when picture styles are used in-camera (eg. portrait, faithful, landscape, etc).
The reason why people use RAW is to perform post processing in software like DPP or Photoshop. This provides the most flexibility to allow the photographer to get the "look or feel" that he or she is trying to achieve. JPEG's can be manipulated in this way too, however there is much less flexibility than with RAW.
I hope this answers your question.... sorry to make it so long.