dtgreene: Actually that's outright wrong. If you look at the first 9 or 10 levels of a typical class's XP table, you will notice that each level takes about twice the experience requirement of the previous level. That is, by definition, exponential growth.
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So, yes, XP requirements *do* grow exponentially.
If you have to change the function to best-fit, it isn't a single function anymore. Yes, for part of the character growth experience requirements double, but for most classes that's only the first handful of levels. Still, you're right that you can select parts of the exp table and find doubling.
Let's see if my memory holds: warrior and rogue double to 7. Paladin and Wizard to 6. Cleric to 5. Druid to 3. That's a lousy small selection to cherry pick from to see exponential growth.
EDIT: Ahh, forgot bards. And forgot to mention that rangers use paladin table. Aren't bards the same as thieves in 2nd?
dtgreene: Also, for Kensai->Mage, why level 12 when just one level will get you the final half attack?
Improved THAC0, mainly. Weapon speed. It's nice to hit first in the round and never miss.
EDIT again: Oh, and nine or twelve levels instead of one because hit points are always nice. 130hp mage isn't shabby.
Final edit for this post, pinky swear: If you really want to do late-game dual-classing, just play 3rd edition. They basically dealt with the complaints about that there. If you want to do mid-game dual-classing, then you should only have a period of a few minutes where you don't have your old skills. If you go to wizard, drop party members, pause, and scribe scrolls like a madman, then tell them you were just kidding when they walk over and ask if you're sure. If you're going to any other class, just clear up to right before the big quest rewards for a handful of places like the Keep (or all the strongholds if you've got that mod), slaver's den, or Aran Livail/Bodhi. Since you have a post three topics down talkign about how you underflow gems to get gold, you don't ahve any standing to call any of that excessive cheesing XD