The thing is cannabis is an annual plant technically. Most grafting is done with dicot perennial woody types, mainly fruits, to increase productivity and to procure the crop that would otherwise not be possible commercially due to problems with hardiness, disease, etc. if the cultivar was grown as a whole. There are very few commercial fruit crops where grafting is not being used ie grapes, apples, peaches, etc. There are also a lot of ornamentals that are also grafted.
I’m not sure I see an advantage of grafting with cannabis other than, as mentioned before, maintaining multiple varieties on one plant as a mother for reasons such as plant counts. But then again to me the whole plant count thing is ridiculous.
Why not breed for better outcomes with weed and when found then use clones. If you want a stronger rootstock, breed for it. If you want to increase yield, breed for it. You want better structure, breed for it. I guess I make this argument because, other than revegging, what do you do with a grafted cannabis plant after it flowers, even if it does slightly increase yield or whatever.