It's actually a pet peeve of mine. I believe it slows things down.

On AA, the FA's will frequently walk the aisle and close mostly full bins. Their criteria seems to be, will another rollerboard fit? If not, then close. Invariably, one or more additional pax will board after that with a backpack, small soft duffel, etc, that will fit, and they have to start opening up bins until they find a closed bin with a small but sufficient amount of space.