Incorporating it with ground soil is a great way to improve the ground soil and poses no problems in my opinion.

Reusing it gets mixed opinions. Some swear never! So do it all the time. So all I can tell you is that I do. I dump all the containers out onto a big tarp, spread it out in the sun with a rake to dry well, then I mix in about 1/3 new mix and some fresh pelleted fertilizers with it. After cleaning all the containers well I refill the containers and go from there. After the 3rd year the old goes into the garden and I start with new in the containers. Never had any problems with this method.

Dave

Here is a link to several previous discussions on the question if you want to get more opinions.

Here is a link that might be useful: Reuse potting soil discussions