Lets try this scenario:

There are 150 people on the plane, its 8:30 at night, the last flight of the night to your destination. You're looking forward to getting home in a couple more hours.

It's time to go, the Captain comes on the horn: "We're all set to go, we're just going to wait a few minutes so that the passengers who just arrived on a delayed flight at gate C-31 can come on board and get home tonight"

Within 10 minutes the door closes, and we're on our way.

Is this an unnecessary delay for 150 people, or a reasonable attempt to accomodate as many passengers as possible? You tell me.