Well I don't know about switching for no reason. I see calculated decisions my my defenders but of course since there is not communication verbally like in real life it looks random as hell. Some of the contextual animations help, when someone points they are literally telling you that they expect someone else to guard that spot of the field. They are covering for each other. If theres a hole in the shape they are trying to fill it. This is based on the number of players between the ball and the goal I think. So I will chase back with a midfielder and fill the hole left by a CB or fullback. Or I can take the FB and user contain and let my midfield catch up/force the dribbler to pass. It's hard to explain without visuals. I'll see if I can record some clip examples when I'm playing this weekend.
If there is no other choice for them to either leave the box or let the dribbler stand with an open shot outside of the box, they are always going to leave the box. The defenders man mark, so if the AI has decided they are the person who has to mark the dribbler that's what they will do.
You can play with the pressure settings to find your preferred spot on the pitch where they step up to defend. I use mine at 60s and make the other team work to even cross the half way line. So my CBs are usually outside of the box. You may be able to set the pressure lower and see how deep they will play. Something I've never tried though.
I do use defensive/ultra defensive when I want them to sit further back.