besides the obvious reason that stationary bowls are a bitch to empty, using a carb hole continues to cherry the bowl and it wastes more weed. you can never really clear a bong with a stationary bowl unless it's on the small side. stale smoke tastes like ass.
some stoned asshole always tries to tip the bong to scoop out the ash and dumps putrid water everywhere. how the fuck are you supposed to empty those bowls anyway?