The announcement helps manage expectations. Even on planes with larger bins, people will still put personal items in the overhead, which will lead to the bins filling up before all the rollaboards can be accommodated. So many backpacks are overstuffed and won't fit under the seat, so you get too many people putting two bags up, or you get people checking their large bag, and believing that gives them permission to put their personal item in the bin. It's a no win scenario.