Before anyone tries to justify the fictional reasons for why ASF works the way it does, it would probably be wise to justify the mechanical argument for its existence. Given the fact that wizards can easily get a very high AC with two or three simple spells (and no movement/skill penalties, to boot) negates most arguments that allowing wizards to wear armor makes them "too tough".
I put an Armored Arcana feat in my PnP game that reduced ASF by 10% per slot, maximum of three slots. That's right. Not even 5%, but 1-0-%. Most arcane casters still didn't take it, and just stuck to the traditional arcane protective spells.
Even with 10% a pop, you're still talking about three feats for the maximum reduction. If you wear almost any medium or heavy armor with a shield, you still have a non-trivial chance of failure. That also ignores the fact that a single-classed wizard or sorcerer isn't proficient in ANY armor. So, unless he or she wants big penalties on top of that, he or she needs to spend a total of six feats to wear full plate with a large shield for "only" a 20% chance of failure. That's one out of every five spells!
I would suggest trying ASF reduction feats out in your game, because they really do open up the type of flexibility that I believe 3E typically supports.