vent holes are also used in tubes to dissipate heat and moisture and alleviate pressure when brazing, but those particular holes are usually filled with brass after the frame is finished. but yes, those holes are to keep moisture from hanging out too long. i have never heard of water "pouring out" from there, though. might want to see if you can find out where that kind of volume is entering the innards of your frame. one suspect is a lack of grease in your seattube, particularly around the "slot" where the binder bolt clamps it down. this will allow water to seep in as it gets sprayed on by your back wheel. run a fender, or put some grease in there. just a thought.