In a normal environment, students IMO should train with a single instructor, if there's a problem with a certain topic, and another's input is needed, that's when they can fly with another.
The other situation is when the student is doing final preparation for check-rides, I would "pass" them around with other instructors to see if they could find something that we were missing mysef due to being familiar with the student. This seemed to be very effective and made sure we brushed up any minor details.
Now in the situation where a student and an instructor just don't fit together well, then that situation needs to be rectified immediately. I had a student that just didn't respect my opinions etc... He had a problem, but in the end it wasn't me. I discussed with him immediately that I wanted him to learn regardless of who he flew with, and gave him options on other instructors we had available at that time. Needless to say we never saw him again, and no one had any hard feelings about it.