First off, look at the original tactical system, magic always resolved first, before missile and before melee actions. Therefore, in the terms of the tactical system being used at the time, a spell that did not require any preparation was basically "instantaneous" for all intents and purposes, even though it was not so in actuality.
The problems stated arising when folks began using different tactical/initiative systems, where spells were not always resolved BEFORE other actions. This combined with the inaccurate naming of the "instantaneous" type of spell made many folks think of such spells as if they really were supposed to be instantly cast.
In RMC, we tried to solve this issue in a manner that makes sense with the tactical/initiative system we used.
We set things so that an instantaneous spell takes 75% activity. 10% for actual casting and the remaining 65% to account for recovery for the sudden surge of magical energies through the casting. This is RMC core, as detailed in RMC Spell Law
Now, anybody who has ever stood up too fast and gotten dizzy for a few seconds can understand the effect we were going for. Basically, casting an instantaneous spell is like that.
You stand up too fast (cast instantaneous spell), and have to wait a few seconds until you are no longer dizzy (recover from the casting).
I hope that makes sense...