The manual says to add the fuse when driving with a spare tire on Automatic vehicles only. It is not a good idea to do this for an extended period of time, otherwise say bye-by to your transmission. You may be able to get away with doing it for a while, but you will be causing unnecessary wear and tear on your car.
And as far as getting better gas mileage, thats complete B.S. It makes no sense at all.
But go ahead and do it anyway. Transmissions can't be that expensive to replace, can they?