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