I would not buy milled product. It would have to be milled over the container that it is being sold in otherwise companies would just mill over a massive screen and collect all the trichomes that fall off during the milling and then resell it to you as a different product that you should have got in one product. I also like looking at the structure of different herbs which would be impossible with milled product.
If I were a dispensary I would potentially order milled to streamline the process of making prerolled joints.
It would be hard to keep the herb at the proper moisture content as well with milled product. It would just dry out too much after it was milled.