its pretty common
almost everyone in australia mixes tobacco with their weed, it stretchs out the amount, say for example you've gotta make a small baggy last for a week..
I used to be strictly anti-mix but then i started smoking ciggies and not long after there was no disadvantage anyway
if i didnt smoke tobacco i wouldnt do it, you'll get headspins and if you smoke alot of it its easy to become nauseous