what?!a clogged filter in a duct system does not keep a motor driving a squirrel cage blower from circulating air through the motor. Most of the blowers I've seen in ventilation systems are driven by motors that are outside of the vent duct anyway. Closing off the inlet of a blower will in fact cause the amperage to decrease because the motor load decreases. Try it and see.
A second thought...If the vent system is clogged the system will probably run for longer periods and more often trying to satisfy the demand...this would likely shorten longevity(years of service) of its componants in general although not shorten a componants given lifespan. (hours of operation)