Increased energy consumption
     With fan power representing 40%-50% of the HVAC energy usage, leakage costs can be significant due to increases in fan speed.

Thermal Loss
     When supply air escapes, variations in temperature between rooms and floors occur, affecting comfort levels in the space.

Equipment Overload
     Your system will run longer than specified to compensate for pressure loss and temperature accuracy; this could lead to increased maintenance costs and premature component replacement.

Code, Airflow, & Safety issues
     Leakage could lead to ventilation and safety code issues related to exhaust, along with excessive power use.