Jerzy Wieczorek, a statistician with the U.S. Census Bureau, explains why the American Community Survey is worthwhile.
Besides the direct estimates from the ACS itself, the Census Bureau uses ACS data as the backbone of several other programs. For example, the Small Area Income and Poverty Estimates program provides annual data to the Department of Education for use in allocating funds to school districts, based on local counts and rates of children in poverty. Without the ACS we would be limited to using smaller surveys (and thus less accurate information about poverty in each school district) or older data (which can become outdated within a few years, such as during the recent recession). Either way, it would hurt our ability to allocate resources fairly to schoolchildren nationwide.
Similarly, the Census Bureau uses the ACS to produce other timely small-area estimates required by Congressional legislation or requested by other agencies: the number of people with health insurance, people with disabilities, minority language speakers, etc. The legislation requires a data source like the ACS not only so that it can be carried out well, but also so its progress can be monitored.