FormerlyHemlock
Hero
I would strike the rule about adding for EK and AT - they equal or surpass single class casting plus still give you everything else.
Wizard 4 vs. F/W 3 (cast 1+3=4) - even
Wizard 5 vs. F/W 4 (cast 1+4=5) - even
Wizard 6 vs. F/W 5 (cast 1+5=6) - even
Wizard 7 vs. F/W 5 (cast 1+6=6) - 1 level below
Wizard 8 vs. F/W 6 (cast 2+6=8) - even
Wizard 9 vs. F/W 7 (cast 2+7=9) - even
Wizard 10 vs. F/W 7 (cast 2+7=9) - 1 level below
Wizard 11 vs. F/W 8 (cast 2+8=10) - 1 level below
Wizard 12 vs. F/W 9 (cast 3+9=12) - even
Wizard 13 vs. F/W 9 (cast 3+9=12) - 1 level below
Wizard 14 vs. F/W 10 (cast 3+10=13) - 1 level below
Wizard 15 vs. F/W 11 (cast 3+11=14) - 1 level below (+500xp)
Wizard 16 vs. F/W 11 (cast 3+11=14) - 2 levels below
Wizard 17 vs. F/W 12 (cast 4+12=16) -1 level below
Wizard 18 vs. F/W 13 (cast 4+13=17) - 1 level below
Wizard 19 vs. F/W 14 (cast 4+14=18) - 1 level below
Wizard 20 vs. F/W 15 (cast 5+15=20) - even
In other words, a Fighter(EK)/Wizard or Rogue(AT)/Wizard is almost always within one caster level or on-par with a single classed wizard in terms of casting, which doesn't balance out for those who want to play a single-classed caster vs. getting all the rest of the cool toys for very limited or no limitation in your casting.
A fighter/magic-user/thief, going Fighter(EK)/Wizard/Rogue(AT) would blow a poor single classed Wizard out of the water by often casting better than them. plus giving everything from 2 other classes.
A few cherry-picked level that show it at it's worst:
Wizard 4 vs. F/W/R 3 (cast 1+3+1=5)
Wizard 9 vs. F/W/R 6 (cast 2+6+2=10)
Wizard 15 vs. F/W/R 9 (cast 3+9+3=15)
Wizard 19 vs. F/W/R 12 (cast 4+12+4=20)
The other casting progressions work, just where you are stacking.
I think you're misunderstanding the OP. The slots stack; but you don't use the PHB multiclassing rules for computing the slots by adding up caster levels. You just add the slots or spell points. Let's take your Wizard 17 vs. F/W 12 for example. The Wizard 17 has 4/3/3/3/2/1/1/1/1 slots (107 spell points) and can cast True Polymorph. The 12th level Eldritch Knight/Wizard has 4/3 + 4/3/3/3/2/1 = 8/6/3/3/2/1 slots (90 spell points), and can't cast anything stronger than Globe of Invulnerability.
The fighter/mage is still a good character, but he'll do more of his fighting with swords and arrows instead of by turning into an adult red dragon.
Likewise, Wizard 19 would have 123 spell points and 9th level spells vs. F/W/R 12's 107 spell points and 6th level spells.
Edit: BTW, I don't think it would cause a mechanical problem if you did silo off wizards and Eldritch Knights/Arcane Tricksters from each other. In practice, those spell points/slots are probably just going to go toward Shield spells anyway. My reason for unifying them is aesthetic--it just makes sense to me that all wizard spell points should be fungible. But if you disliked that extra complexity you could eliminate the exception.
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