A 3/4 caster would only miss out on 9th level spells, I've felt that a 2/3 caster would fit well in the game and top out at 7th level spells, I actually have a spreadsheet somewhere with the spell progression. I'd have probably preferred the bard on a 2/3 spell casting track with more class features over making them a full caster.
Unless I'm mistaken, a 3/4 caster would go up to 7th level spells, so they'd miss out on 8ths and 9ths. While I generally agree that that's not
too much of a reduction in power, it could still be made to work. You'd start off with 1st level spells, and gain a higher spell level at levels 4, 6, 9, 12, 14, 17. This means you'd pull consistently ahead of the half-caster at level 6, and fall consistently behind the full-caster at level 7, which seems like a pretty good place for the three types to diverge.
I don't, personally, see all that much utility from adding any of these things. But it seems to me that, if you really "had" to add something, a 3/4-caster would arguably be more distinctly different from the half-casters than a 2/3-caster would be, while remaining distinctly different from a full-caster. It also better splits the difference between half- and full-casters, capping out at 7th level spells (where half-casters cap out at 5 and full-casters cap out at 9). The only real benefit I can see to a 2/3 caster is that the progression table is smoother: at one more than every multiple of 3, you gain a higher spell level, until you cap out at 6.
If half-casters didn't exist, so the only models we had were full-casters and 1/3-casters like EK and AT, then I could see 2/3-casters as fitting into a nice niche. But with both 1/3- and half-casters present, 2/3 just seems to compress too much into too tight a space: people cap out at 4th level spells, 5th level spells, and 6th level spells...or go all the way to 9ths.
Edit: It's worth noting, I was presuming that there would be an artificial cap on progression the way that full-casters are artifically capped off from getting 10th level spells like they're supposed to mathematically. (That is, full-casters gain a new spell level at every odd character level...except 19th, where they get nothing. I was assuming similar limits for both 2/3 and 3/4 casters, blocking them from getting what
should be their highest theoretical spell level.) If that cap isn't considered, then yes, 2/3 casters and 3/4 casters would go one spell level higher at extremely high levels. I personally find that sort of thing weird, and thus wouldn't design something that way--why have features that are only reached if you literally get to the highest or second-highest level possible in the game?