It's a dreadful UK habit to put tobacco in a joint. I used to do it for years, but the last fifteen years or so it's been all pure. The argument about stretching weed doesn't hold water. If you are only holding a small amount just hit it in a pipe. You don't get any higher by making the smoke last longer. Put it this way. If you have a decent whisky, a single malt, would you piss in the glass to make it last longer? If the answer is no then why fuck up your weed? If you actually like to mix then fair enough, but doing it for reasons of economy doesn't make sense.