<p>For standard ABS and PLA filament, most distributors recommend storing the filament in an airtight bag.  Does not doing this actually make print quality worse?  I have left mine in the open for a year and have had no noticeable problems.</p>

Humidity may be the problem.
Humidity tends to degrade filament, making it weaker.
If you leave a coil of filament out, over time it will be exposed to humidity. I have yet to hear of this happening over a short period of time - the real threat comes if you leave it out for weeks or months - but it can happen nonetheless.
Contamination with other materials is possible but unlikely. The odds of some sort of impurity developing from nearby particles is extremely low unless the filament is actively exposed to some other material.
In most cases, though, things should be just fine.