Vents are prone to accumulation of dust, dirt, grime, and biological matter from rodents that live inside. Over time, they increase, and all the soil and harmful pathogens in the ducts get circulated into the living space. This causes health problems, such as breathing troubles and asthma. It can also cause diseases which are due to pathogens transmitted by rodents.
Moreover, large debris and dust in the vents increase the stress on your air conditioning systems, which makes the work harder to circulate air. This translates to higher electricity bills. It is better to clean the ducts occasionally, and even though it might cost you, you might see savings in the long term.