The main reason the distinction exists is flavor. Someone felt a need to sharply distinginuish between arcane and divine magic. That isn't true in all games. In GURPS Banestorm, all magic is is essentially arcane, with miracles being a different and less predictable phenomenon. Clerical casters are essentially priests who also happen to study magic. In Warhammer, the distinction is less than in D&D; each type of caster has their own list, but there are Lesser spells they have in common, too, and each wizard order is different from th others, as well. Runequest divides things up by casting style (rune, spirit, or sorcery), not by arcane versus divine. 1st and 2nd edition Talislanta had all casters, clerical or arcane, using te same spell lists, while 3rd edition and later gave each Order of magic its own spells. So it really has a lot to do with campaign design.
As pointed out, if you took away arcane casters' unique access to the most powerful spells, you have to give something in return. The easiest thing to do would be to eliminate clerics, such that traditional clerics are essentially fighter/wizard types. Druid would become a PrC that granted spellcasting progression and offered wildshape and their special immunites. You also have to sort out what level spells become available at. The easiest thing to do would be to say that for spells not appearing on other lists, they are available at the same level if the classes have the same number of spell levels. If one class has 4th level spells, adjust the spell level up once (for five, six or seven level classes) or twice (for eight or nine spell level classes).
