Long story short, pressure will build up in the crank case which is not good... even though you're vented to atmosphere, having the intake "suck" the air out of the crankcase (creating negative pressure) helps the engine in efficiency and reliability. Basically; pressure in crank case = bad for rings and seals,,,,, blow-by gasses and oil in crank case are also bad and won't be evacuated as efficiently as having a vacuum on that tube.

Here's a good/quick read to give you a slightly longer version of what i just said:
http://en.wikipedia.org/wiki/Crankca...ilation_system