Rev hang - a curse or blessing

[aquasurf]

Wouldn't that lean out the fuel? More air without increase in fuel?

I do not think so. You can possibly lean out a fuel mixture, that is to change proportion of air and fuel in it, but if you have no fuel, then you do not have a mixture to lean out. You have just air and it ain't no pollutant.

Edit: Initially, the injected mixture would suddenly lean out as fuel supply was being shut off and emissions would spike. This is the very reason why car designers try to avoid it by adding the rev hang.