We do allow a large amount of home-grow, in particular for medical patients, with an extended plant count. Unfortunately, our data is not able to distinguish whether a poison centre call or an emergency department visit is related to homegrown or store-bought. I would certainly agree with Mr. Vandrey that a regulated market is going to be safer from that standpoint.
Those individuals who are growing at home and making their own concentrates and edible products are very likely fairly experienced with it and less likely to get into problems over having too much.
The last point I want to make is this: where is the product that more naive users are using going to come from? The more availability there is on a regulated market, the less likelihood there is of that product coming from a black market, where it may be much less clear what's in it, and it is potentially more dangerous.