The history of spontaneous casters is not born with 3E; rather, long before 3E arrived, I used to see several netbooks with alternate casting systems for magic-users in them; I may be mistaken, but I think Dragon Magazine had one or two alternate magic systems in its history before even 2nd edition. I also know that, as far back as 1987, our house rules used to include spontaneous casting of ALL clerical spells for clerics and druids, because it made sense as "miracles called by the priest" and because it never seemed to unbalance the gaming play. I even ran an al. wizard in my 2E games in the early 1990's where you could cast ANY spell known to you once, but if you wanted multiple copies, they had to be written down ahead of time. This encouraged them loading up on the magic missiles, fireballs, etc. but they saved 1 or 2 slots at each level for the esoteric things like glitterdust, or detect magic, etc. I would be surprised if thousands of other DMs didn't have similar experiments.
I have a feeling that all this history was well known to the designers when they sat down to make a draft of 3E...