Whilst it annoys me when I see the pictures of people evacuating with the their hand baggage, I do understand why this happens.
Of course, as cabin crew, we know what we are doing and we go through the preparation day after day after day, and every year go through our recurrent training etc… But the passengers do not. They don't think about these sorts of things and being as some will fly once a year, maybe a few more, it is to be expected.
In our training, we are told that, if by the time they get to the door, they have their hand baggage, there is nothing you can do. Because firstly, it will waste time and as already pointed out, you need to get them out. Secondly, and this can be quite dependant on type of aircraft, you have no where to put these bags if you take them off someone. For example, at the front of the narrow body Airbus family, you would have no where to throw the bags. However, at D1 on a 744, you have all of A zone to fling the bags.
However, saying all of that, as I mentioned, if they turn up at the door with their bags, too late. Shove them out the door.