It makes just as much sense as a character who has been beaten down to a single hit point being able to fight with equal efficiency as a character who is at full health.

Or a character who's a Grandmaster in fighting with a Mace, but is completely clueless as to how to use a club.

Or how a dual classed character "forgets" everything he knew about his old class until his new class exceeds it (or... in the PnP rules... loses ALL experience for that adventure if he does fall back on his old skills).

Or how there is absolutely no difference whatsoever between a character with 7 Dexterity and 14 Dexterity.

Or how druids can use a sickle because "it's a farming tool, that could potentially be used as a weapon" -- but hammer and axe are completely out of the question.

If you're looking for an RPG rules system that "makes sense", you would be well advised to stay FAR clear of 2nd Edition. :-)

I could offer my personal views, but they would just be anecdotal. Different worlds and systems had different rules, so I don't know how comprehensive an answer can be gotten. I can offer something about the "universal" rules, which you may find helpful.

In the early editions, clerics could only cast up to 7L spells (where wizards could cast up to 9L). Those levels were broken down as follows:

1-2 = gained by faith / meditation
3-4 = given by divine servants (devas, etc.)
5 = given directly by god (demi, lesser, greater)
6 = given directly by god (lesser, greater)
7 = given directly by god (greater)

IOW, if you worshipped a demigod, you could only get up to L5 spells, while if you worshipped a greater god, you could get up to 7L spells (5-7 still being given directly)

These breakdowns affected ingame casting. If the cleric was somewhere in the multiverse that didn't have access to the astral plane, they couldn't get 3-4L, as the servants couldn't reach them. If they were in another "Power's" desmesne, they might or might not get any god spells, depending on the relations between the two Beings (which would also affect the servant spells). So if, for example, a prime plane cleric of a god travelled directly to the home plane of an enemy god, they would only be able to recover spells of 1-2L, because the "home" god would block the "visiting" team. This would put clerics visiting the plane of hostile powers at a significant disadvantage.

It should also be noted that casters were "charged" with power when they memorize the spell, not when they cast it. This is how a cleric could enter a hostile Being's plane and cast powerful spells initially, because the spell power was stored in them... they just couldn't recharge those spells. Bear in mind the devs were making D&D from scratch. It was the first RPG system ever, they had nothing to go on but their own imaginations and inspiration from fantasy authors. Thus, it's obviously going to be imperfect. Then, once the game became popular, the devs were reluctant to rewrite the entire magic system. There were, however, countless suggestions by gamers and devs over the decades for different systems, which people could use if they wanted.

When video games came along they started out using the memorization system that D&D had, but eventually moved over to the mana system. Video games, being one-off productions, are far better able to create new and improved systems than an RPG that's been around for decades. And of course even there D&D has changed, being on what now, a fifth edition?

Currently, I think video games favor the cooldown system. Cast a spell, and then you must wait a bit in realtime to cast it again. No mana or memorization.

The levels of spells are like the level of your character... they're numerical ways of showing how good you are at something, how powerful. We know that a 10L fighter is much tougher and more dangerous than a 1L fighter, and the numbers give us a sense of how much. Think of belt colors in martial arts... it's a form of ranking.

I hope this helped you, and if I can help with anything else, feel free to ask.

Does this mean that there could be a situation where a cleric could get level 5+ spells, but not level 3-4 spells?

As a side note, I happen to like the arcane/divine spell split from a mechanical perspective (it allows there to be two different types of casters, one focused on healing and one focused on destruction), but not from a flavor perspective (why does one need to be religious to get healing magic?).

This post actually made me think of a different idea for a magic system, one that might work for a Druid-like class. A character with this type of magic would get their spells from nature. The catch is, of course, that the spells you could prepare (as I have mentioned before, I prefer this term to memorize in this context) would be based on the terrain and environment where they prepare the spells. Create water would be a nice spell in the desert, but it can't (usually) be prepared there; the spellcaster would need to go to a more moist environment (like a forest or by a river) and prepare the spell there. Once in the desert, the spell could be cast, but to regain the spell inside the desert, they would have to find an oasis or hope that it rains.

Going further, perhaps certain powerful spells would require specific locations. For example, maybe getting resurrection magic requires that you find a specific World Tree and get the spell there; furthermore, a character can only have one copy of the spell prepared at a time. (This is actually a Dragon Quest reference; in the series (starting from 2), there is an item called the Leaf of the World Tree (Yggdrasil Leaf in newer translations) that will revive a character when used; there is generally some respawning source of the item in the game, but the item won't be there if you already have one. (In Dragon Quest 4, there's an actual World Tree dungeon, but if you try and take a leaf when you already have one, the leaf "won't come off".))