Which is my issue.
At 20, multiclassing isn't really a problem. You can make a character at level 20 in a vacuum and not have any problems with it.
Of course, I can't even remember the last time I played a game at 20, let alone to 20.
The game has very clear tiers, not a smooth curve. If you get yourself behind tier progression then you find yourself having perhaps several levels essentially stuck in the prior tier.
Of course, you can make a character at any level past 10 or so and have multiclassing work out alright. Making characters in a vacuum is easy and everything is sunshine and roses.
The problem is in actually playing a character, level by level, while attempting to multiclass.
This is why I wonder why the multiclassing spell chart exists. More than 1 or 2 levels of multiclassing is difficult to keep up with in various tiers.
Seriously, who cares if a Wizard dips into Cleric for a level and gets 2 Cure Wounds spells? He's pushing his gain of next level spells back a level multiple times in order to heal 10 or so points of damage per day.
Why should someone who is Cleric 10 / Wizard 10 be a threat with 30 spells from levels 1 to 5 as oppose to the 22 spells for the level 20 Cleric or Wizard? The extra spells are almost all level 1 to 4 and the PC loses a ton of awesomeness from the missing 6 6th through 9th level spells.
Spell level, straight caster, even split multiclass caster at 20th level
1: 4 8
2: 3 6
3: 3 6
4: 3 6
5: 3 4
6: 2
7: 2
8: 1
9: 1
Yup, with a non-restricted model, the multiclass PC can spam a boatload of lower level spells. He should be able to do that. He paid for those spells with multiclassing. He gets a lot of spells, the straight spell casters gets the cooler higher level abilities and the really powerful high level spells.
I totally do not get this one.
Ooohhh. At level 20, I can fire off 16 Fireball spells a day. Course, I'm facing creatures that have 300 hit points and make most of their saves and can suck up most or all 142 D8 of those 16 spells (which take forever to cast).
Even at low level (multiclass followed by straight):
Level 2: 4 first vs. 3 first
Level 3: 5 first vs. 4 first and 2 second
Level 4: 6 first vs. 4 first and 3 second
Level 5: 7 first and 2 second vs. 4 first and 3 second and 2 third
Level 6: 8 first and 4 second vs. 4 first and 3 second and 3 third
At level 2, the multiclass PC gets an extra spell. But, he is giving up the second level special ability of one of his classes to do so. At level 8, he gains a lot of spells, but it's 4 first and 1 second vs. 3 third. Not really a contest.
Not seeing the issue. The multiclassing PC gets a few extra low level spells once in a while, the straight caster gets an extra highest level spells each level (except levels 12, 14, and 16 which also seems weird).