Taking time with a student who is not at the level of the majority of the others in an ensemble is important for a couple of reasons. First, to help the player gain confidence and get positive feedback from the others. Secondly, to build tolerance and patience in the rest of the players.
Depending on the situation, the player who is having trouble keeping up because of not being as advanced as the other players could get more out of being with others closer to their own level. Especially if it is creating a lot of stress on them and the rest of the group is getting very frustrated. This can happen more easily in a larger group such as a band or a large like instrument ensemble where there are lots of people playing the same part.
If the students are keen and the teacher or conductor has skill working with people, there is an opportunity to observe and learn what can be done in those situations where someone is struggling. Now, what if the reason for the person struggling is unpreparedness? How the coach, conductor or teacher deals with that is also a good educational experience for the other students. If a person really is not taking the time to do their work, how much allowance should the teacher have? What is the breaking point? I am starting to rethink this in my own teaching. I can have a huge allowance but is that always useful? I think not. But, when should a person (student) realize that their development is up to them? I would say in general terms, the earlier the better!
At the college level the allowance does shrink even for me. But I always am examining how I can broaden my perceptions. That is why it is very important for me to understand what the person's reasons are for wanting a career in music. Plus, what do they want and how much are they willing to work for what they say they want. I realized when teaching at a few state universities the goals were different (from conservatory students) in terms of how much a student really wants to improve on their instrument. Now that is generally speaking of course. Some end up being terrific performers on their instruments. Many of the students I have taught at the state university level were interested in teaching/conducting high school, middle school or elementary school bands. I would encourage these students to practice a bit more and push themselves so they even have a better understanding of what it takes to advance. This will also help them deal with playing situations and challenges they will come across with their students because of how they learned to deal with their own issues. The more a person keeps going regardless of the ups and down they will encounter, the more they can see and understand others because of what it took for them to get to the next stage in the process of their growth.