The reason they usually puke is too much gas fumes in the cannister.  The charcoal get saturated and falls apart.   With out the cannister, you may get fumes in the car or garage when the car is sitting.

 

The cannister absorbs fumes from the tank.  The tank must be able to vent or it will build pressure or vacuum as it heats and cools.  It must also allow air in as you drive or it will create a vacuum and starve the engine for fuel.

When the engine runs above idle by not wide open, it applies vacuum to the cannister to pull fresh air through and dry out the charcoal for the next time the car is shut off, and needs to vent.

 

If you seal the line to the tank, you must use the vented cap, and most of the fumes will not be noticed when driving, but it won't make any difference when parked in an enclosed space.  If the line is left vented toward the front of the car you could get fumes when driving.