I'd rather read what everybody else has to say and learn from others' experiences with other guns, but I must ask: don't heavies have access to pretty much as much of the map as everybody else does? I thought they just had to take longer routes around and that there were only a handful of places only lights can get to.
In a game where chokepoints=failure, being able to get around chokepoints is one of, if not the, most important caveats you can have access to. It's certainly worth much more than 2 extra hits from the most common gun in the game, or an extra few seconds of hilariously inaccurate fire.
Just as important is the light's speed, which can grant him a relatively easy out of combat gone bad or a method of circle strafing a fat heavy to death.