I've done lots of Cannabis grafting and my take is there is not much to gain from doing it but it is a lot of fun. Grafted plants always attract attention and are great conversation starters if you have visitors to your grow.
First of all, Cannabis is very easy to graft when the plants are young and the stems are solid, flexible and non-woody. A bit more difficult as the plant ages, stems become woody and hollow. But overall, easy as far as grafting goes. Cannabis is considered an herbaceous plant, i.e. non-woody, even though it eventually develops distinctly woody stems. Herbaceous plants are, in general, easy to graft.
I would suggest an approach graft (
http://www.burkesbackyard.com.au/fact-sheets/in-the-garden/gardening-tips-books-techniques-and-tools/make-more-grevilleas) if you are grafting young plants. As NaturalFarmer suggested, Parafilm is your friend if you are grafting. I'll post more detail on approach grafting if there is some interest but the Grevillea page pretty much covers it. The only thing I do differently is to wrap the grafted area with parafilm.
I have also tried side veneer grafts. Side veneer is the most widely used graft in the ornamental plant world. For example very named variety of Japanese maple was probably propagated by a side veneer graft. This page has a complete rundown of grafting. Take a look at the photos of side veneer. All you need is a VERY sharp blade and parafilm. Side veneer is the graft of choice when grafting a small rooted cutting or seedling to a mature, woody plant or carcass.
https://content.ces.ncsu.edu/grafting-and-budding-nursery-crop-plants.
I've also tried plug grafts but the hollow stems of adult plants allow plugs to dry out too quickly and success is limited.
As was suggested by someone above, grafting a young plant to the carcass of a harvested plant does produce rapid growth initially but in my experience a rooted cutting growing on its own matures just as fast and gets to be about the same size as a grafted cutting.
Photo one: <a href="
http://s46.photobucket.com/user/emulsion1/media/CIMG0070_zpsftszvupy.jpg.html" target="_blank"><img src="
http://i46.photobucket.com/albums/f107/emulsion1/CIMG0070_zpsftszvupy.jpg" border="0" alt=" photo CIMG0070_zpsftszvupy.jpg"/></a>
Photo one shows an infamous Herbies 'Afghani' that was harvested then used for my initial grafting experiments.
You can clearly see a healed side veneer graft just above the highest clothes pin. The two clothes pins are holding a poly bag in place over a plug graft. As a side note, these Afghanis always turned purple for me but the change came mostly after harvest.
Photo two: <a href="
http://s46.photobucket.com/user/emulsion1/media/CIMG0071_zpsdjmrgtfm.jpg.html" target="_blank"><img src="
http://i46.photobucket.com/albums/f107/emulsion1/CIMG0071_zpsdjmrgtfm.jpg" border="0" alt=" photo CIMG0071_zpsdjmrgtfm.jpg"/></a>
Photo two shows a close up of a healed side veneer graft. You can see the 'veneer' of the stock (Afghani) plant to the right of the healed graft. The veneer and skin of the Afghani are dark green or purplish-green. The grafted plant which was Serious Happiness, is pale green. You can also see parafilm wrapped around the stem of the Afghani above the healed graft.