I'd pay the premium to know that I'm not getting something laced with rat poison or PCP.
After some arguing and googling "weed smells like pine", he came down from his paranoia perch and relaxed.
There was a news report in the UK a while ago about people adding glass (or something like), IIRC it was well substantiated.
Say the dealer laces his pot so a twelfth of an ounce actually weighs an eighth. Unless the additive somehow plumped up the pot's volume, the buyers would look at it and say 'you're ripping me off - that's a really light eighth.'
Sure, the dealer could then bust out some scales, but that would be weird. Not to mention dealers love repeat customers and glass in the mix would put an end to that quick.