It's not rare. The potheads are losing this long time debate favorite. It's 30% now, down from 35% in '01-'02.
Anytime you start messing with neurotransmitters and brain chemicals like GABA and dopamine, all of these having significant effects and feedback loops, you are going to find addiction. You don't simply block important receptors and your body not adjust to your drug uptake by adjusting all the other pathways, thus creating a dependence on that drug to maintain the balance (which is artificially and out of whack in hyper and hypo production of other chemicals, enzymes, binders, etc., even if balanced).