As Welfare Comes to an End, So Do the Jobs
New York City has largest number of people working for their welfare benefits, and nowhere else in country are so many of them reaching cutoff at same time; because of slumping economy, many of those enrolled in city's workfare program are being laid off rather than being hired as regular employees; workfare is elaborate mix of work assignments, job training and job searches; its champions call it way for welfare recipients to give back to city and as path to real jobs for many; critics say it has been dead end for many poor people in need of education and training, way to cut and divert people from public benefits and create tide of cheap labor that has swept thousands of paying jobs from city payrolls; program provides evidence for both arguments; photo (L)