This depends heavily on what you mean by work. If by work you mean a minimum wage job at a McDonalds or Walmart, that isn't exactly what I would call purpose and dignity. For a meaning of work that included intellectual/artistic exploration without a guarantee of economic return on investment, but excluded economically viable but degrading labor, I could agree.
That's one way to look at it. I'm not disagreeing with it, but another way to look at it would be - any work that puts food on the table is work with purpose and dignity. Yet another (self centric?) way to look at it would be - if the job that I am doing helps me grow intellectually, emotionally etc then it is good work (regardless of whether it is useful to others/society or not).
May be these questions would be more important in the future, where most of the boring/mundane jobs would be left to the robots. Then people can spend more time to work on things they like, and less time to work for money.
Fairly certain that most people would disagree with "any work that puts food on the table is work with purpose and dignity." Easy examples come to mind: prostitution, arms dealing, etc.
The thing about usefulness to society is that we just don't need everyone to be useful anymore. Society will continue on perfectly fine even if half of the population contributes nothing at all, because of technology.
The future is not so far away anymore, and societal change is very slow. These are the kind of questions we need to be thinking about now.
