Hmm.. interesting point. It always annoys me too.

The need to switch perspectives between people makes it easy and interesting to find out about what is going through the minds of all involved. However; when the person switches constantly without warning.. well you get the idea they didnt read it through when they were doing it So if writers want to change perspective i think something like ~Now from ......'s point of view~ is needed to make it clear.
Not just switches randomly.