The problem Rand is that example is merely a very simple to lay out one that people get. . .
Often, with my high level Paladin, I made the comment of "You attack me, you're in trouble, but if I have time to buff spell myself for 3-4 rounds before I attack you, you're screwed."
Which becomes "I buff myself 5 spells at the drop of a hat." so you're always screwed.
Like, what kind of counter are you going to use on someone who starts combat by dropping multi blade turns, multi deflections, Anti magic radius, radius attack, radius attack.
It starts an arms race not rooted in tactics of action, but in closely reading the spells and stringing up 5 hit (or 10 hit if you allow 10 x 10% spells) combinations.
Haste, +100 OB, Extra Crit on next attack, Extra Crit on next attack, Extra Crit on next attack, Attack 100%.
Paralysis, Paralysis, Paralysis, Lightning bolt, Lightning bolt. (2 LB at 0 DB is going to slap you up)
My playing crew, handed the capability to cast 2 spells per round, turned to SL to find the best 2 shot. . .stringing 5 would almost cause brain damage in the potentialities it opens.
two gets bad enough when the caster is doing bladeturn/attack every round. It's probably not a game broken imbalance at 2, but it's pushing in that direction. . .keep in mind that this started off, all I said was "Instant cast items are no problem, the only time I see it become a problem is if you also break the 1/round limit." time to cast is a modification, casting 5-10 spells a round is game breaking. . .no idea why that comment was controversial enough to prompt so many replies.
And there's nothing about it being win-win all the time, it's about how it way shifts the balance in favor of casters to the point where you simply cannot ever allow a caster the option of choosing to attack, simply because they are unstoppable combinations in any normal situation. You shift the paradigm to where there's only 3 ways to ever beat a caster: 1) Eliminate magic in the area and attack them (like a cancel essence spell with a radius) 2) Ambush them and kill them before they react or 3) Run them out of PPs then kill them.
Magic is powerful, and great, but when you tip the balance to the point where magic trumps anything else, you break in play balance. The problem also lies in the fact that it's not simply the caster/non balance you're breaking. . .any caster can then kill any other caster with a combo, so it's a matter that you've broken the offensive/defensive balance so far toward offense that defense becomes almost impossible.
And saying that "Well, the demi god living under a mass contingency isn't threatened" or "The giant living in the non magic zone is safe" are very narrow exceptions to a generally game breaking scenario.
GoF, I think it was in Dragon magazine, but they suggested taking a canvas bag, going over a PCs equipment list, and dropping a bunch of approximately shaped objects in the bag. . ."I draw my wand out of my bag!" "OK, draw the pencil out of that bag while I time you." Then penalizing activity accordingly.