It doesnt make the bike feel much lighter at all... However, your front brake is overkill in terms of power for most dirt jumping situations. Add to that the fact that a front brake complicates bar spins and tailwhips and such and you either need a giro or you need to run the cable through the steerer tube to avoid it getting tangled and many riders just feel that it's not worth all the hassle when it's not a truely necessary component for that aspect of riding.
