Many of the people that use Opiates find that it is a body high (obviously) and more of a relaxing state of mind. Once you get into that fuzzy, relaxed state, that's what you like to feel. Mild opiate users have that relaxed state mentality when doing opiates, and that relaxed high you get from smoking weed.
Once you get into heavy opiate use/addiction, that's all you want to feel. Weed is a (normally) relaxing or up feeling. When you use H, but smoke weed by itself, you normally feel that it isn't giving you that nice feeling, and most users tend to dislike that feeling. That's normally what happens when you use opiates, and why most people have such a hard time quitting.
EDIT: Whoops, forgot some... And of course, mixing both H and weed gives you that relaxed feeling that comes from H, so that you can really feel normal. Smoking weed on top of feeling relaxed usually puts you in a VERY comfortable high; Hence why people like mixing them, and not going back to just weed.
Although, as a mild opiate user, i've found weed can help you quit if you can tolerate it for a while alone.
