I agree with Violin Star - we are not a charity! Why would you do this for free? I will go further and say (controversially I presume) that in doing it for free you are devaluing other professionals and making it harder for them to earn a living. I feel this about teachers who massively under cut the going rate for lessons too - perhaps someone who doesn't have to make a living out of teaching, who has a partner who supports them etc.
I understand your point, but I really don't think it's the same thing. This is my sole income and I do not have a husband/partner to support me, and yet I fully stand by my choice not to charge my own students for accompanying. It is very much a business choice.
A better comparison might be teachers offering refunds and make-up lessons for cancellations. After all, we are not a charity - why should any teacher offer a refund for a missed lesson? When someone does so, aren't they undermining the profession? Now, I personally do not offer refunds, but it's something that has to be up to each individual teacher to decide. Or what about when a teacher gives a student 15 minutes extra because they are struggling the week before an exam? Or the teacher who spend an hour on the phone to a parent who is concerned about their child? Shouldn't they charge for that hour of their time? Or a teacher who spends hours creating materials or sourcing music for one student? What about teachers who put on concerts for free? What if I took my students to perform at an old people's home - do I charge for my time then? Etc etc. Every minute of our time is valuable, and technically worth paying for. Yet no teacher truly charges for everything they do. We all have to decide where that line is for ourselves, and it's never going to be the same for everyone.
We all do more than teach someone for 30 minutes a week. Parents might see it as paying for a half hour lesson, but in actual fact we set our fees so that they cover both lesson time, and everything else we do for our students in between lessons. Some of use include exam accompanying in that "everything else", and some don't. As far as I'm concerned, it's part of what I offer; part of what they are paying for when they choose me as their teacher. If you see differently, that's fine, but it does not mean that I am devaluing your job. It's not the same as setting yourself up as a teacher and charging half the going rate - I agree that's not on.
As another example, lets' say a shop offers a free book with every purchase: does that mean they are devaluing all the other bookshops? Does it mean they are acting as a charity? No, they are making a business choice; that actually they believe, in the long run, their business will be better off for offering that book for free than they would be if they charged for it. That actually they will make more profit despite giving away that book. In the same way, paying for lessons with me comes with "free" accompanying. It's not devaluing - if something had no value, offering it for free wouldn't mean very much! It's only considered a "perk" based on the fact it is actual worth something, and would need to be paid for elsewhere.
As I've said, when I do other accompanying work - for those who aren't my students, or for my students but not on the instrument I teach them - I do charge. I know what my time is worth. Specifically, I know what my time as an accompanist is worth. But I choose to make this part of of what I include for my own students. I also make sure I don't lose income by taking time out to accompany my students. If I am unable to rearrange my teaching so I can be there, then they would have to find another accompanist, although so far that's never happened.
Taking this approach is the best way for me, at the minute. That doesn't mean it's going to be the best approach for anybody else. As I said in another thread, we all have to do what is going to work best for ourselves and our students. I think, like most things, this is an area where we have to make the choice that's right for us.
Now, having said all that, when someone is doing general accompanying - not for their students - and charges little or nothing, then that I do see as an issue! I know there is someone who has done a lot of exam accompanying in my town, who charges something like £10. I'm not sure whether that's for a rehearsal and the exam or whether he just doesn't rehearse with them at all (which seems shockingly common), but either way, it makes my own reasonable rates (definitely not the highest locally) seem very high. I'm not seeking large amounts of accompanying work, so it's not something that's caused me a major issue, but I know I have lost potential income to him, and it does bother me. So I do definitely sympathise/agree with what you are saying, I just see what I choose to offer my own students as a different matter.