Alistair Bell mentions that an inline fan can impede airflow if the fan is
off.  On of the advantages of the JCWhitney fan for Vgons is that there is
little of this effect.  The fan is centered in the duct, and the fan's
housing is at least the diameter of the rest of the duct, and the air does
not have to turn a corner as it would with a squirrel cage.  I wouldn't want
to have to run the fan all the time, and I don't.  Not that a Vgon is quiet
enough at 70+ so you'd NOTICE one extra fan.  
B.