Mediations, as with so much of life, involve relationships. I do believe that mediating with all of the important people present is a best practice. Insisting that anyone personally attend should not be made in a knee-jerk fashion however. You may actually do yourself, and your client, a favor by not “insisting”.
Professional courtesy can be valuable, in the case at hand and in those yet to come. In addition, if you insist, you may have to wait for the mediation to occur on their schedule, not based on your preferences.