??? I'd guess so bugs can get in and use it for their winter home ????
Never really gave it much thought, but if the plug is out, there's a better chance that it will evaporate any water left standing below the drain plug. I always use my wet/dry vac and a piece of windshield washer hose to vaccuum it all out and then put the plug back in. Whenever you do put yours back in, make sure to use teflon tape on it or it will corrode and be much harder to get out next year when you winterize. (that is assuming you have the metal plug with the anode rod attached.)