This thread is interesting to me. I definitely get the use cases.
I worked at pro labs in Toronto doing image assembly in the '80s.
We made multiple exposed internegatives so enlargements could be made from them.
I wish I could remember the model of IR viewer. It was really heavy on the head, and we needed to wear them a lot.
It didn't happen to me, but my colleagues reported that they sometimes got shocks from the crummy cable on it. Yikes!
We had to remove the negative from a pin registered easel and remove the lith film masks in between exposures to change transparencies and make changes to the enlarger.
It's a bit foggy in my memory, but I had done this at another lab previously. At the first lab, we didn't use an IR viewer. It wasn't a hardship, but I did like using the IR at the next job.
The biggest job I was involved with was 21 exposures, made across 2 8x10 internegatives.
Fun stuff, and so was all the prep. required to make the layouts, and lith masks. Lots of overtime.
My modest darkroom has no need just yet but I can see using one for loading film etc.
Cool thread.