They burn the unburned gas in the exhaust manifold big deal. If the
engine is tuned up correctly to start it will add nothing.  As far as
wasted horsepower I expect by disconnecting it the fuel saved would be
about a balance with the increase emissions saved thus no actual  net
decrease in emissions.  I have actually went through the emission tests
with the belt off and failed due to the belt being off but if you look
at the emission numbers the emission was better with the belt off then
on.  On my truck the pump gets driven off the alt. so adding wearing it
out prematurely.

Unless it is a vehicle that is fully electronic with  fuel injection all
the other emission junk is just that junk.