After reading this answer, I was once again struck by the fact that a Dungeons and Dragons 3.5 spellcaster, no matter his level, is unable to overcome the limits imposed by his ability scores.
Like most classes that cast spells, the paladin, for example, has this sentence in its description of the class feature spells:
To prepare or cast a spell, a paladin must have a Wisdom score equal to at least 10 + the spell level.
Thus a paladin, no matter his level, is unable to cast spells if his Wisdom score is 10 or less. He can be a level 20 paladin, commander the holy and orderly hordes, lauded by all good and organized folk, feared by evil and chaotic creatures throughout the planes, his god's bloody right hand of vengeance and wrath, yet, because his Wisdom is only 10, he can't cast spells.
How can a DM explain this limit to a player when the player wants his character to be able to cast spells, but the player is, for example, either unconcerned with his character's bonus spells and saving throw DCs or more concerned with an accurate statistical picture of his character than the mechanical benefits a different picture would yield?
For example, how does the DM explain in a narrative fashion that a character must have a high Intelligence score to realize fully the wizard's spellcasting so his concept of an addled, not-so-bright but patient and persistent wizard is invalid? Likewise, that a character must have a high Wisdom score to realize fully the cleric's spellcasting so his concept of an oblivious, judgment-impaired but dedicated and devout cleric is also invalid?
Mechanically, why does this restriction exist? That is, are there technical reasons to tie a character's maximum spell level to his ability score directly rather than, for example, to his class's standard spellcasting progression? I'm not looking for developer commentary (although that's great if it's available) but for game elements that make this connection necessary.
Does, for example, a wizard need an Intelligence score of at least 10 or does a paladin need a Wisdom score of at least 11 for anything else besides satisfying that one line in the description of the class's spells class feature? Does the game for some reason descend into anarchy if, for example, a Wiz17 with an Intelligence score of 6 can cast 9th-level wizard spells or a Pal15 with a Wisdom score of 3 can cast 4th-level paladin spells?