I don't know what events you attend, but this has almost never been the case for me or any event I attend. Outside of maybe a wedding for obvious reasons (which you don't want to just slice a piece off the wedding cake) I go to events that's never been an issue.

If anything you would it maybe had a point for the opposite reason. Events I go to you need to cut a piece out so that you actually have a chance to get a slice period. If you had said do so to ensure you get a piece of your own dessert then I would have agreed, but I see no issues with folks wanting dessert in my kind of events.