I don't have an opinion on this one other than to make mention of my bad imagination which is struggling to think of how one can end up accidentally modifying tags under the Tags (2) section of the tag editor (on top of the save-prompt).
Has this happened to you before?
Twice!

I also fear others in my family might (and may have already).
The first was a fluke - I added changed some tags using musicbee and it did a rescan/wrote the old tags to the new tags, or something similar - things were a mess! I learned not to mess with tags they are being rescanned and to be careful to do things tag related one step at a time.
As far as pane 2 goes - mine is full. I used it to manage: releasetype, itunesadvisory, ensemble, library (I use this to break my music up into Christmas, etc), copyright, discog link, etc.... I also use it to hold tags I only want to view but I am frequently in there tweeking data.
To be fair and to your point - it is a rare occurance - but when it comes to upc, isrc, source id, etc. I can't imagine ever wanting to editing them, so why risk it? I have never needed a seatbelt to save me and hope I never do but I still wear one. In any case I'm.just looking for readonly access to some tags. I am suprised my OCDer homies haven't already asked.

Side note: I play with my metadata alot.