Something else that makes it a less practical is that you are either extending your brewday in terms of things you have to clean. If you are keeping it in the kettle, you have to clean it 24 hours later. If you are transferring to another carboy, you are cleaning that. At any rate, you are extending the commitment of your brewday.
I find it a lot easier to manage a starter leading up to brewday than adding new wort to a vessel of already fermenting wort and managing the risks involved there. If I screw up a starter, all I have to do is start a new one. If I mess that up, I just flushed a batch down the drain.