If you plug all holes up the moisture will work its way in and have nowhere to go damaging the frame over a longer period of time. With no hole in the bb shell the water entering through the seat tube/along the seatpost will just go straight through instead of collecting and structurally weakening the frame later on. BB and crank axle can get ruined as well. Many new frames already have cutouts or holes. Drenching a new frame with Framesaver/Linseed oil is a good tip to prolong its life.
If you plug all holes up the moisture will work its way in and have nowhere to go damaging the frame over a longer period of time. With no hole in the bb shell the water entering through the seat tube/along the seatpost will just go straight through instead of collecting and structurally weakening the frame later on. BB and crank axle can get ruined as well. Many new frames already have cutouts or holes. Drenching a new frame with Framesaver/Linseed oil is a good tip to prolong its life.