@
Maxperson
For example, you seem to forget the things pointed out to you when you refer back to 1e, everything from the basics (like how proficiency worked) to the more advanced, like how the rule/fluff mechanics worked together.
Once again, we will use the MU as an example.
Why can't a MU wear armor?
Because-
"Furthermore, they can wear no armor and have few weapons they can use, for martial training is so foreign to magic-use as to make the two almost mutually exclusive."
That's right- MUs can't wear armor because they lack .... martial training.
So, what happens if you have dual-classed, human F/MU?
"The character may mix functions freely and still gain experience,
although restrictions regarding armor, shield, and/or weapon apply with regard to operations particular to one or both classes. ... [Using the example of a fighter/magic user,] furthermore,
the character can now carry (but not wear) armor and weapons
not normally usable by magic-users, and resort to their use if the need arises and not be penalized in respect to experience as a magic-user, for he or she has already surpassed in the new class the disciplines of the former."
PHB, p. 33
Woah... wut? That's right, EVEN IF the character trains up and gets martial training, the character still can't wear armor while doing MU stuff (like casting spells) because REASONS!
Because the bit of fluff you keep looking at (spoil magic powers) for the druid is the same as the bit of fluff added to the MU part by Gygax- just extraneous verbiage, not as important as the actual tables and rules.
The MU couldn't wear armor because that was a class restriction. The Druid couldn't wear metal armor because that was a class restriction. Any attempts to retcon with a more modern thinking would necessarily fail.
(To be clear, the rules were not consistent- if you were a demihuman, the rules for edged weapons and armor fell aside, but the their armor rules stayed in place, also because of reasons; the influx of UA just made the bad rules worse)