FFXIII is a special case though, almost every previous FF has alternate methods or strategies to getting through the game, as well some alternate means of getting spells/skills aside from experience (which tends to be why bosses & certain rare enemies/areas don't give exp in quite a few FFs). S-E would have to re-do the entire crystarium ala FFX-international to encourage low-level play in FFXIII. On the other hand there is a No-Crystarium-Usage walkthrough on gamefaqs. So eh. S-E figured that if players wanted the game to be more difficult, they'd discover their own methods (which is what they've done for every previous FF).
Level isn't all that important for casual FFXII play anyway. I think part of the reason for Weak mode's existence is that vanilla FFXII offers an item that keeps the player from gaining EXP, but it isn't available until quite a ways into the game. It's more of a convenience-thing (plus it frees up the accessory slot).
