Most leaders in most organizations feel obligated to continue the practice of mission statements. Before we understood brand the way we do today, mission describes the organization's (possibly unique) value to their markets, in other words, its market purpose. It was supposed to communicate purpose to employees, stakeholders and markets and guide all decision making.
Reality is, if your employees, stakeholders and markets are uncertain of your market value, you don't have a mission statement problem. You have a whole different level of problem. And if your decisions are so simple that one brief statement could serve to make them, you have a seriously high risk view of your world. If people are freaked out by not having the perceived security of a mission statement, have one. Just don't wrongly expect that having one will solve the problems of brand confusion and decision making intelligence.
If you don't squander a lot of time trying to craft the perfect mission statement, one that wins everyone's approval, you have time instead on the far more significant questions about vision crafting and brand management.