I still do not think that the general problem is that of beginners or more experienced users.
I can think of several occasions where more than one instance of MP3tag do not lead to any advantage:
Examples with no claim for completeness:
Instance 1 has a filter, you edit in instance 2 so that either the filter in view 1 matches (again) or does not match - expected behaviour?
You have loaded a folder in instance 1, edit files in instance 2 so that the files are moved to the folder that is displayed in instance 1 - expected behaviour?
You start an action in instance 1 and start another action in instance 2 and the actions lead to the result that files cannot be handled by the other action as the e.g. replacing strings do not find a match any more. Is that as you expect it?
Files are usually not blocked so that the last edit wins... and so on. MP3tag is "the universal tag editor" - so it is all about editing, not merely viewing.
I found that iTunes or WMP display complete libraries just as nice and allow to list certain files e.g. to keep the names of artists consitent. I then transfer that list to MP3tag and edit the tags there. For me and my workflow, there is no need to have further instances.
(ah, yes, the updating: WMP frequently updates its library if you edit files with MP3tag - this leads just as frequently to locked files. And iTunes does not do anything like it. So you have to do it manually (or with scripts) which - for me - leads to exactly the shortcomings I described at the beginning.