There can be many challenges in maintaining long-term studies, often without funding.  However, these challenges can be out-weighed by the benefits that can be mined from the field or existing data collections.  Long-term studies can offer generations of researchers the unique opportunities to examine the affects of treatments over time without having to wait their entire carreer for the opportunity.  Researchers can take a fresh look at data collected over time, or collect new data from studies started well before their carreers began.  Though sometimes overlooked and under-utilized, they can be a look into the past that may preview the future.