You would gain... Sewer gas explosion.
Never should sewer vents be exposed to any ignition source...
In mines it's known as firedamp and it kills indiscriminately. All it takes is the proper ratio of flammable gas to oxygen and you have a fuel-air explosive.
Sewer gas is vented to the atmosphere for several reasons:
it displaces oxygen
sulphide gasses are toxic
if allowed to accumulate, it is explosive
For more information on what you're playing with, NACHI (National Association of Certified Home Inspectors) has this nice page.
