If you raise your baby from the start to learn to settle, then it would never get to the point of leaving them to scream and vomit.
If a child continues to wake and wants to sleep in your bed, when you don't want the child there, then this can be rectified.
I would never advocate leaving a child to scream and scream. I would always go into a crying child but not immediately. Sometimes a child will settle after a short cry. Other times the crying might not stop so easily.
The thing to do is go in, so the child knows they haven't been abandoned but do not reward them in any way for waking and crying. Have a set phrase like, it's sleep time, night, night and then leave the room. That way the child has had reassurance that the parent is around but they haven't been rewarded for the unwanted behaviour.
If parents are unhappy with the child disrupting their sleep then it's necessary to take steps to rectify the situation. It's nothing to do with tough love or being firm, it's about changing an undesirable situation so everyone can get some much needed sleep.