I've played a barbarian in both those kinds of campaigns too. In a campaign with 6-8 encounters the low-level barbarian is extremely lack luster. Sure, it makes a difference comparing the barbarian from the one campaign to the barbarian from the other. It just doesn't unbalance them wrt the other classes.
Your details support my point, I am unsure how you come to a different conclusion. Can we examine?
In one, the barbarian was lack-luster. An at-will, like a rogue, would be the same all of the way through.
In the other, the barbarian was not lackluster. An at-will would be at exactly the same.
So, if in one the barbarian is worse and in one they are better, they absolutely vary compared to the static value of the rogue. So their relative power compared to the rogue varies. They aren't both balanced against the rogue, so it affects balance between classes.
No one in the campaign with fewer encounters was saying, "wow, your barbarian is too OP". And that campaign was a majority of martial characters.
So a 2014 beastmaster ranger is balanced against all other classes and subclasses because no one says it's "too OP"? No, balance has to do both when it's stronger and weaker.
If the barbarian is noticeably weaker, it's out of balance.
I feel like you're white rooming a little.
I am, this is a generic discussion talking about averages. It's somewhat unavoidable when talking about the state of the game as a whole. You are right that there can be individual variations, but looking at the math can give you a big picture feel across a lot of campaigns, and is what you need to design against.
Sure, if there had been a second wave those spells would have been very useful.
Which was my entire point.
You don't necessarily know at the start of an encounter that reinforcements are coming. If you do, sure, casting a spell like spiritual guardians is a great idea. OTOH, if you cast it and it turns out to be a 2 round easy fight with no reinforcements, you've just wasted your elephant round swatting a fly. If you wait a few rounds to see whether reinforcements arrive, then you've wasted a large chunk of the spell's value.
I feel you are getting stuck in the weeds. I talked about a spell you would cast for your average 3-4 round combat, how
if you already cast it, you would get extra utility out of it if the combat went longer. By this point, you have already cast it for the encounter you knew about.
And again, duration is the small part of the whole thing. Don't get too hung up on it. It's 20% of the whole presentation.
Finally, no, Hypnotic Pattern is not providing you any more value when the second wave arrives. It CC'd whatever enemies it did from the first wave, and that doesn't change when the second wave arrives. It provides no more value than if you used it in two separate encounters. It has no capacity to hypnotize the second wave, and for you to even attempt hypnotizing the second wave you need to release concentration on the first. Furthermore, if the second wave disrupts your concentration and the first wave is released from the spell, you're getting less value from the spell than if you'd used it in two separate encounters.
Wave 1 has 8 foes. Hyponotic Pattern catches 6 of them, 2 whom are later broken out. 4 foes are killed leaving 2 active before wave 2 appears with 3 more foes.
Case 1: You fight five foes (2+3) at once, with two still caught in Hypnotic Patten to be dealt with later.
Case 2: You believe Hypontic Pattern brings no additional value and drop concentration. You fight seven foes at once.
Not all duration spells bring extended value, but even on the one you insist can't, it can. Case 1 is better than Case 2. It doesn't have to, and maybe it will be best to spend more resources and cast a new concentration spell. But that's more actions and resources, and the apples to apples of just that one casting shows there is still residual value in the Hypnotic Pattern.
And that's one of the least, while the Spirit Guardians and Spiritual Weapon will retain most/all of their value when a second wave comes.