I (like a very large plurality, maybe even a majority, of devs) do not work for a consulting firm. There is no client.

I've done consulting work in the past, though. Any leader who does not take into account (at least to some degree) relative educational value of assignments when staffing projects is invariably a bad leader.

All work is training for a junior. In this context, the idea that you can't ethically train a junior "on a client's dime" is exactly equivalent to saying that you can't ever ethically staff juniors on a consulting project -- that's a ridiculous notion. The work is going to get done, but a junior obviously isn't going to be as fast as I am at any task.