For Americans who can’t find jobs, the booming demand for temp workers has been a path out of unemployment, but now many fear it’s a dead-end route. With full-time work hard to find, these workers have built temping into a de facto career, minus vacation, sick days or insurance. The assignments might be temporary — a few months here, a year there — but labor economists warn that companies’ growing hunger for a workforce they can switch on and off could do permanent damage to these workers’ career trajectories and retirement plans. “It seems to be the new norm in...