Can someone explain this to me? Because I really don't get it.
Oh, I guess I get part of it - levelling both classes feels more organic, like you're really both things at once.
But (using the 5e exp table), how is a 12 level single-classed character possibly equal to a 9/9?
Keep in mind the major process for multiclasses under AD&D...
You maintained separate XP tracks for each class.
– High PR bonuses to earned XP were specific to a given class.
– Class specific bonuses went to the specific class.
You maintained (Classes +1) different HP maximums. One for each class, used only for what was rolled. The last was the average of the rest, and was the one used in play.
– This made low level Fighter Wizard comparatively frail, next to single class fighter... but rather buff compared to single class wizards
You got the benefits of all classes, but not all the drawbacks.
– less restrictive armor was used
– Less restrictive weapon set was allowed
Starting Gear was the better of the two classes
You STARTED multi-classed
I think that last one is rather key... under 3.X and 5.0, everyone's single classed until at least 2nd level. Under AD&D, starting multi-classed meant levels 1-3 were easier. Levels 4-6 you were comparable to a 5-8th level character in power and total XP... but had fallen behind the HP curve enough to really matter.
That 9/9 character had hit points of a 9th level PC. He was more potent, combat wise, than any one 9th, or even 10th level PC... but he dies like a 9th level. Meanwhile, the 12 single has fewer but stronger abilities than the 9/9... also, THACO and Saves were better of each, but didn't stack. That F9/W9 is 250K XP in each... total of 500K ... 1 XP from W10, and having enough total for wizard 11 or fighter 10.
And using the old exp tables, it's generally worse. (Human characters were quite rare back in the day for that reason. And I don't think I EVER saw a single-classed demihuman.)
I've seen a few. I've even played a few, when the stats didn't allow for multi.
And in campaign play, not a few players opted for single class in order to gain HP at a decent rate.
And if you're multiclassing two spellcasters, what happens when you hit 20 levels total? How many spell slots do you have? Or is this a case where level limits come back?
I suspect the 5E solution is levels 21+ won't add to spellcasting, won't add to HP, and won't add to Proficiency Bonus, but will add more features and possibly attributes.
And, based upon what's there in the 5E PHB, assuming the character has stuck to "bard, cleric, druid, sorcerer, and wizard classes", his spell slots will be the same total as a single classed 20th level: 4x1 3x2 3x2 3x4 3x5 2x6 2x7 1x8 1x9. If he's instead a Pal 10 Wiz 10, he counts casting slots same as a 15th level wizard (Full wizard + half paladin), but maintains separate wizard and paladin spells memorized.
It's actually simpler than it looks, once one realizes that all the "full spell users" (to borrow a Rolemaster term) are using the same slots available table, except the warlock.