Most RPGs use classes of some sort to determine what skills a character has access to.
There is a roleplaying element to this as a knight is not suddenly going to start learning spells.
On the other hand a mage should not be learning new spells out in the wilderness for no reason at all. Of course this depends on how different a spell is, a mage with experience only from fire spells might have problems figuring out how to do a frost/ice spell on his own but may be able to figure out how to use fire in a new way on his own (especially if it goes from "fireball" to "bigger fireball").
Finding a scroll would be fine, but that is a limited way (in a game that is realistic about were and how often they can be found). Teachers would be less limited in what can be learned but they are limited in where the learning can take place.
Alignment was/is a system to determine how a character should act in the world and to a degree what classes the character has access to. This has a problem built in only if the DM/GM enforces staying in that alignment.
To make it more interesting alignment could be used to determine whether the character gets access to teachers that can teach him/her abilities.
Paladin would be a good example of how classes could be handled based on a dynamic alignment except for that a paladin who strays from the path of his god (such as lawful good maybe) would not have access to his faithbased abilities any longer. Perhaps knight would work better. Lawful knight if lawful and chaos/chaotic knight if chaotic.
