Jon:

The vent tube on the crank case is important in keeping dust out of the bottom end of the engine. While the net flow is out of the crank case, there is a small amount of reverse flow when the piston is rising. With a minimum of 4" tubing on the vent the reverse flow doesn't have time to draw any dust back to the inside of the engine.

And as Matt said, it keeps the engine compartment cleaner when it's pointed down and out.

Bill.