I spend a lot of time thinking about spell-casting. I'm not really the biggest fan of the Vancian magic system that is used in AD&D. Not to say that I don't understand it's purpose, to limit spell-casters, but it seems rather unwieldy overall. Or maybe it's not so much unwieldy as the explanation of it is dull and unimaginative.
I allow spell-casters in my games to free-cast and to use memorized spells. Free-casting takes longer because the whole spell has to be carried out with all of it's requisite components. Not something done in haste. Yet by allowing free-casting, it offers the spell caster greater variety and specificity in the spells they do cast when they need or want to cast them. That makes the game more fun. I refer to free-casting as the "Live" part.
I also allow for preparing spells ahead of time. This fits into the Vancian system because it can be described as memorizing spells, especially in the case of Magic Users and Illusionists. It can also be described, at least I describe it so, as "spell-queuing". This fits well with Clerics and Druids who don't obtain or cast spells the same way.
Spell-queuing is having the spell-caster go to their place of preparation. This might be a portable altar, meditation area or something along those lines. In the case of M-U's They proceed to memorize the spells they anticipate needing to use in the coming day. For Clerics, They will perform the meditations and prayers to have their pre-selected spells Ready To Use. I like to describe it as performing the entire prayer or incantation but leaving out the very last word or action to execute the spell out. Then, when on the spot use is called for, they say that last word or perform that last action, etc... and the spell is executed nearly instantaneously.
If the spell-caster is about to knowingly go into a battle or enter into a building or dungeon, etc... where they think they might need to have certain spells at the ready and no time to go through the whole casting process, then the "Memorex" approach can be very very useful. Though I will usually judge that spells which are "Memorexed" lose a bit in accuracy and other specificity. In other words, they sacrifice fine controls for expediency.
During the battle or the exploration they can still go "Live" and carry out the desired spells with full accuracy and fine controls but it costs time in the execution. Either way, "Live" or "Memorex", the spells count against their daily allotment.
So for Players considering this approach, I suggest you think of the situation the PC is looking forward toward. If it's going into battle, what kinds of spells would be highly effective but usually not able to be gotten off because they take too much time to execute? Those are the spells that you "Memorex".
Basically, casting time is a general deciding factor in which spells to "Memorex" because if it takes more than a few segments to cast, then you aren't likely to get it off in the midst of battle or in an emergency situation.
"Cure Light Wounds" isn't worth "Memorexing", it's low level and relatively quick and easy to cast and it's usefulness in an emergency situation isn't high. Cure Serious Wounds, Cure Disease, and the like, those are worth "Memorexing because sometimes poisons and rapidly accelerating diseases don't allow for a lot of time, maybe just a round or so, to go through the full process of casting. Being able to whip out "Slow Poison", "Remove Curse", "Cure Critical Wounds" or spells like those is much more worthwhile. When you have just a few segments in a matter of life or death, you don't want to have to wait longer if you don't need to.
I do pretty much waive the casting time for "Memorexed" spells because the casting was already done at the preparation. The execution is instantaneous from the uttering of that last word or action. Time to take effect is still subject to the spell descriptions. For "Live" spell-casting though, I enforce casting time limits entirely.
There you have it, another peek into my weird world, deranged and out-of-whack as it may be, it sure is a lot of fun.