One very useful and versatile item that is routinely omitted from kits of this ilk is an elastic bandage (Ace). In practice, over the years a 2" or 4" Ace has been the most used item in my kit, either for myself or some one else in the group. I remember one week long backpack where all six ace bandages were eventually wrapped around someone's dinged joint; we looked like the "Spirit of 76."

That is one specific instance of why I like to assemble my own kits; usually you can save a good bit of money as well, as well as keeping good control of the quality of the critical item on which you may well depend.